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.

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 you can be.

I like 10 levels. More new stuff per level and the time spend on a level gives better familiarity with the classes

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.

I really liked the 18th level capstone and standard subclass level progression from the first playtest. We just hit 11th level in my game that we have been playing for 2 years. It's so hard to hit those highest tiers.

I like 10 levels. More new stuff per level and the time spend on a level gives better familiarity with the classes

I do see where you're coming from, but I think that basically just makes 18 the new 20. If you're already going to 18, going to 20 from there isn't really a stretch, but you're less likely to want to if you already got your cool capstone at 18.