Wow, you guys are really crapping on CapnZapp for sticking to his guns, it must make you feel like reasoned genial people. I also think it's against forum rules.
I've played to 20th twice now in 5E. Full casters and their insane versatility and power are very hard to account. I've been the DM once and the player once, and there is no avoiding catering everything to the specific spells your PCs can cast of 8th and 9th level. I can handle the fighter swinging his sword really fast, Meteor Swarm and Wish are a bit harder to not break the fiction handling. All that aside, it seems unlikely that WotC actually intended to play beyond 15th level because they could have easily extended any of their larger adventures to include all levels. There is a reason they didn't, accounting for full caster magic 15+ is a headache no one wants.