The more I read about 5E, the more I get the feeling the later designers at Wizards had little idea what they were doing. Tasha's book broke the game by giving too many ways to break the bounded accuracy system. I get the feeling the design team was responding to the popular sentiment that "Missing to-hit rolls sucks."
So, they broke their game to fix a design flaw. Only the rules are so vague that different groups could disallow the parts that break the game and others won't - leading to wildly differing accounts of system balance and how easy the game is. Overall, the game is still far too easy, and if you use the broken parts, it is laughably easy to survive almost anything.
They fell into a design pattern of "patching complaints" rather than "proceed on solid design principles."
Conversely, games are using 5E's flaws as a marketing point. Pathfinder 2 has universal, unambiguous, agreed-upon rules that are the same for every group. MCDM RPG eliminates to-hit rolls and actively markets against the "negative experience" that D&D forces upon you. How will this work out in practice or will the entire game devolve into "total versus total?
Which reminds me of another game...
Tunnels & Trolls, published in 1975, the second RPG ever published, never did melee to-hit rolls (only ranged to-hits, and in the 8th edition, those have been pared back). The idea is familiar if you know your history. Then again, only some creators want you to learn history, so you must click on their YouTube videos to get their manufactured version.
But a lot of 5E was just piled on as books were released, and since the entire community has this sort of "who cares" sort of attitude, every group allows or bans different things, and the thing we like to call D&D isn't one codified set of rules we can point to. It is a blob with a million items stuck on it and floating around inside, different from every group that plays the game.
Nobody plays D&D because nobody can say what it is. There isn't a single accepted set of rules and options everyone plays by. When people say 5E dominates the market, I can easily say a million versions do, and each has a minuscule market share.
This is a colossal stealth problem for a VTT, which will force one way of playing on everyone.
Nobody talks about this.
You get 10,000 players who love a Pokémon-style battle monsters version of D&D from a 3rd party publisher, and yes, they are counted as 5E players. But, oops, the VTT doesn't support that! There is no good way of tracking and leveling our battle monsters in the VTT. So we don't use it. This example uses a 3rd party product, but even for people who 100% use Wizards content, there still are so many interpretations and styles of play they will never fit into the VTT system cleanly.
And even if a substantial amount of people are willing to pay and play in a VTT, how many will keep doing it after three months? A year? The investors may expect millions; if the company delivers hundreds of thousands (which would be excellent for any company), they may fail to meet targets. The Dreamcast sold 10 million units in North America and was sold out for a year, but it was still canceled since it did not meet an insane expected sales target and user base.
Then you understand how much money it will take 3d artists and content creators to sustain a market like that, especially if the assets are in-house. If you aren't allowing 3rd party set, figure, animation, and content creators in your VTT - you will never be able to sustain a beast of customers that hungry.
And if people don't get what they want - they leave quickly.
Other companies will be doing what you tried to do better because they can organize a team around producing results in line with customer expectations. The ones you failed to meet but paid all the money to develop the market for them. I've been in companies like that, too.
I worked in commercial 3d, and I know you can wreck the health of a talented team if you try to support a ravenous community hungry for content all yourselves. You aren't building a walled garden; you are making a marketplace. Keeping this in-house due to greed will cause it to crash and burn.
Evidence? How long can you sustain a video game development team in all crunch-time mode? The level of work developing assets is similar. Primarily if they must be "coded" to work with game rules. I have been there. They have no idea what they are getting into.
And investors aren't patient, giving you time to figure it out.
Pathfinder 2 goes hard in the other way. The rules are very well-defined, and there is no room for interpretation or group customization. Pathfinder 2 is what you play after you get sick of how undefined and random D&D becomes. I hate telling players no, and I hate having to chase down every exploit and keep ahead of the latest cheats; no, you can't bring that third-party book into my game; this is broken, that sucks, and everything is so random and undefined it should all work out but never does.
When I get sick of the D&D "whatever this game is," and I want to play an actual game with clear rules as written? That is Pathfinder 2.
The character sheets are still atrocious, but I appreciate what they did. The game is not for me.
Level Up Advanced 5E is a rules fork of the original three 5E books. It is math-compatible with NPCs and adventures, but it creates a new sandbox for character creation and advancement incompatible with 3rd party classes and subclasses. It would be best to build your character using these rules to play this game as intended.
The disadvantage is that very few of what sells 3rd-party books are usable, such as the character options. This is also the advantage since many of those, even the ones Wizards put out, are so broken to hell that they are not worth putting in your game. So Level Up does its own thing and puts out regular 'zines' and collects them at the end of the year, and there are some 3rd party expansions, too.
So, while 3rd party character options need to be converted and homebrewed to work, there is good support overall and likely more than I will ever use with the gazetteers. If I want better base compatibility with 5E subclasses, Tales of the Valiant will probably do better in this area.
You can import straight 5E NPCs, and they will work, but try to play them as PCs, and you will discover they are missing subsystems and pieces that A5E needs to make their exploration, social, and skill mechanics work correctly.
A5E is easier than 5E to play since it only requires a subset of multi-book knowledge and is a fork based on the core rulebook engine. To enjoy it, you have to enjoy the goals they were aiming for, which are rules that support combat, exploration, and social encounters with mechanics. It is 5E in a bubble where things are balanced and work well together.
ToV is the game with better across-the-board 5E compatibility.
A5E is a custom hack that brings in the best of 3.5E and 4E.
Owning many 3rd party books I use to put a game together, I am very picky about what I allow in my bubble. I came into 5E at the end of the game's lifecycle, so I am happy with the basic book classes flavored with a few options. I can homebrew in special powers for classes, backgrounds, origins, and ancestries. D&D has a problem with people constantly getting bored with what they have and always wanting to consume more. This is a problem with a game too focused on delivering many options with very little quality to each choice.
To me, the story matters more.