I don't see them ever doing an explicit ".5" edition ever again. Doing an implicit one, such as Essentials for 4e, sure. But there was backlash when 3.5 wasn't as compatible as originally stated. They had no option but to stay the course, but I don't think we'll see that for marketing reasons.
But yeah, I should have said the earliest I could see a new edition is the 50th Anniversary. And you're right that if they don't do it then, it's not for a while after. But I'm not as convinced that the suits won't push for a 50th Anniversary edition.
Maybe 5.5E + errata is how they will do a 6E. We don’t have to scrap everything to create new edition. They didn’t the first couple of decades of d&d. And most other rpg’s don’t do that either.
Sorry, I meant something in the sense of Rules Cyclopedia for Basic, Player's Option for 2e, 3.5e for 3e, Essentials for 4e, etc. 10 years into the edition having a fully-compatible rules revision and update, a republishing of the core rules with all the errata, the fixed Ranger from Tasha's, replacing races with Tasha+ ancestry guidance, removing alignment from monster stat blocks, etc. Very little new for us who have bought everything since 2014, but just compiling the core rules changes that have happened along the way and publishing it in a re-release of the Hydro74 alt-covers.
6E will be something quite different, taking years of feedback surveys like 5e had, reassessing core rules assumptions that can't be changed for 5e without a full edition change. For example, the Fighter having a subclass "Champion" with no story difference from other subclasses, just mechanically is the autospam/caps lock Fighter? That might be replaced in a 6e so that all Fighters have dials from simplicity to complex outside of the subclass mechanic. Maybe all complex fighters use Martial Superiority Dice, while all simple fighters simply get bonus damage/critical ranges, etc. Maybe all Martial characters can access tricks that function like spells and don't require "DM May I" while all magic users can dial down and just spam cast if that's what they want too.
Maybe Sorcerers cast entirely using a spell point mechanic (kind of like a 3.5e Psion).
These are just examples off the top of my head that would be hard to retrofit into the core game.
There would have to be VERY hefty rules revisions needs to release a full edition shift. 5e sells hugely well, the DMs guild is a major profit source and churns out tons of content that WotC can't get to, and there are tons of side-games like AiME, Anime5e, LevelUp, etc that build off the 5e rules because it's for another year the overwhelmingly dominant ttrpg.
I just don't see an edition change happening before 2034.
I DO see them doing something VERY special for 2024. And keep an eye out for an in-house version of D&D Beyond + roll20. I wouldn't put it past Hasbro to want to cut out the middle folk in time for the 10th anniversary of 5e and 50th of the brand.
