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 some of WotC's design philosophy with 5.5 is about encouraging faster progression so that more groups do get to 20.
However, their focus on revamping the PHB, rather than the DMG, leaves the onus for balance on the DM, which has IMO always been the limiting factor. Designing encounters for level 17 players is wildly different from level 16 players. If capstones come at 18 and represent anything close to the power jump that 9th level spells provide, that's two back-to-back power jumps in very close succession. IMO that would actually drive more DMs to end at 17, rather than 18, or even sooner to avoid reaching those T4 power jumps at all. If they delay their BBEG encounter until those powerups, they have little to no experience with their group's new abilities to accurately balance on the knife's edge of very deadly, and risk either a TPK or a floppy conclusion if they miscalculate. "Easier to balance in T3, and end there" is, I believe, still the status quo.
Now, if after the PHB rewrite they make it easier for DMs to balance T4 encounters in a DMG rewrite, thst might change things. In that case, spreading the power jumps out still makes sense to me.
Even faster progression?!
My inner grognard already thinks that 5e levels characters up way too fast. The rest of me enjoys most things about 5e, so it's not a big deal. When I DM, I can use milestone-based leveling and dial it to my preference.
Here's hoping they bother to write some fucking adventures that go above level 11. Faster progression isn't going to matter if they don't actually support the gameplay in any way, shape, or form.