Capstones are rarely acquired in actual play since next-to-no groups manage to get to 20. Even published adventures tend to end at 18, or see very little content beyond 20. Having capstones at 18 would enable more groups to be able to actually play with these cool superpowers.
Epic boons may feel a bit flavorless compared to features handcrafted for specific classes, but there isn't much point if no one is playing with them. And even if an epic boon isn't tailored to a class, they can still make a significant impact for those few who do make it all the way to 20. I see it as a reward for the victory lap, rather than finishing the race. It would even give impotence to the concept of running a 18-20 campaign, and I could easily see myself running this kind of game.
Fighter, Paladin, and Sorcerer have subclass progression beyond 18, which would disrupt WotC's shift to re-standardize subclass progression. But a simple nudge of those features could easily be done in a mere side-note added to the new handbook without disrupting game balance or adding much complexity.
I can't argue against there being a definite novel righteousness about a class' big superpower coming online at 20. But I feel that the impact of shifting that spike to 18 is both healthier for the game and not as detracting as one could expect.
I'd love to see a complete rollback on the decision to standardize subclass progression with the 2014 version, but that's a whole separate can of worms.
Does anyone disagree? Are there merits to the 20th-level capstone that I haven't accounted for?
I think that a martial class's biggest ability should be universally available at level 17 because that's when casters get access to 9th level spells.
I also think that pure casters should not get big class-abilities beyond level 3 or 4. Their 7th, 8th, and 9th level spells already are huge game-changers. They don't need to double-down for no reason.
Then at some point you will run into another problem - later in the game, all casters besides maybe sorcerer feel the same. Yeah, you have class features, but when you have a choice between casting high level spell or using class feature you will choose high level spell in 9 times out of 10. Also, most of the casters subclass features are enabling certain playstyles instead of giving new OP ability.