Li Shenron
Legend
And I get it might be disappointing to hear that... but at the end of the day, it just forces a person to decide what is really more important to them... moving to 5E because it's the one currently supported and can find the most willing players (even if a lot of the rules don't match their preferences)... or sticking with their preferred game so that they can play the game they want-- but just have to put in a little more work finding new players and creating/adapting new material to use for their game.
Really liked your summary, but with regard to this last bit, I must say that personally I feel I can play several different games I want, using 5e core rulebooks, and not just one. The key is to treat the core game more as a toolbox and less as a religious book. Sometimes Crawford & pals actually remember about this and tell us so in their "interview" videos or posts, but most of the time they still act more like they're telling us the mandatory way to play the game. But the core books have A LOT of stuff that probably the majority of us aren't following religiously in our games (only in our theoretical discussions), and by changing the focus on one aspect or rule somewhere along the line between "key feature of our game" and ignoring it completely, you can definitely steer the game towards many different styles.
One example could be how much you use Inspiration and how often you grant (dis)advantage by narrative descriptions done by the players: change these habits, and the game can swing heavily between a more narrative style and a more technical style.
But then indeed, some things that depend on numbers can't be changed easily at all. If someone considers having two-digit bonuses an essential element of playing "3e style" or non-linear ability bonuses for playing "BECMI style", then clearly 5e doesn't offer these options.
I find it hilarious that people think 5e is modular already (according to the terms laid out during development). It clearly isn't. If modularity is defined as having some optional rules in the DMG, or variant race rules, then yeah, tons and tons of games would meet that definition and it loses all meaning.
When people have this discussion I take it to mean as whole collections of rules and adjustments that you remove and add together, you know, in a module. So you would have a narrative one, a tactical one etc. Which, also happens to be what I think the developers also meant when they talked about it during Next.
Yes but if someone expects a "module" to be an entire book, or at least a beefy chapter, perhaps it's because they are underestimating how many rules are needed to actually make a difference. You don't need a book for a "called shots/permanent injuries" module, you can probably have that in one single page. You can make BIG changes to how your game plays just by changing the standard duration of short/long rests.
It cuts both ways. Lots of people disliked Tasha's "options" because they felt WotC was dictating them rather than presenting an option. Imagine if they had printed the Greyhawk Initiative in Tasha's; people would be furious that WotC was changing the initiative rules on them!
Honestly, WotC ceded the ground on modular rules to 3pp like Level Up because any option they put in would be considered Core by the majority of players who would either demand to use it or rail how WotC was changing their game on them.
It's a no win situation
It's primarily a failure of the gaming community, which is on average way too obsessed by what is the "current official rule/version".
It is also a failure of the game designers, when they push something to be less than optional, where actually almost everything in the game could be optional by nature.
Take something like passive checks. In theory the PHB goes quite light with them, the text is more about suggestions. And yet there's a couple of specific abilities (maybe a couple of feats) which refer to passive checks and would lose some meaning if a gaming group decided not to use passive checks. But then the DMG goes down in a more patronizing way about how the DM actually should use them by default for secret doors and such. The failure of the designers here was not to recognize how the gameplay style changes: use passive checks regularly and the game is a lot more predictable outside of combat, ignore them and it get a lot more random. How is that not very much a gamestyle preference that should be left to each gaming group? Why was it not more presented as a "dialable" part of the game, like for example group checks (which have similar power in swinging the overall feel of the game outside combat)?