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?
Personally I’ve actually played at level 20 far more than I’ve played at level 18. I’ve never reached level 18 in a standard ongoing campaign, but I have played level 20 one-shots.
Besides, so many of the existing capstones are honestly rather lacklustre. It’s not that interesting to get “thing you could do before, but a bit more often” as the reward for putting all of your effort into being the best <insert class> you can be.