I don't expect to see 6e any time soon, for the 'goose laying the golden eggs' reason listed above. And beside, while I haven't played all the way to level 20 in 5e, as far as i can see there's nothing cripplingly wrong with the system yet. Part of this is obviously due to the slow release pace and resultant relatively small number of player options compared to previous editions, which reduces the scope for broken stuff to slip through the next and PC design becoming a separate system mastery minigame. But the basic maths is holding up in a way that (for example) 3e failed to do at high levels.
There's stuff that I think 5e could do better, naturally. Aside from issues with individual spells or subclasses (there's always going to be those), there's a few issues around bonus actions, Str is borderline worthless for anyone who's not a Str warrior, there's too few points at which you can make character customisation choices, the Warlock is too one-note, polymorph is painful to use (as it has been in every edition, to be fair), a few skills badly need tidying up (has anyone ever gotten meaningful use out of Medicine?), Inspiration is pretty wishy-washy and could use a boost and possibly alternate 'plot twist!' type uses alongside the +1d6 default, the race/lineage/etc issue needs to be resolved, and so on and so on.
But all this is pretty small bikkies, to be honest, and the cost of developing a 6e to address these issues is arguably higher than the cost of just leaving them alone. WotC (if they're smart) are probably very wary of putting out a 6e which is just 5.5e (let alone an actual 5.5 or revised PHB...), after the ... controversial ... release and reception of 3.5e or 4e Essentials. I reckon they'll probably just continue releasing optional rules in upcoming 'Everything' books for a while yet, and let individual groups take what they want. I wouldn't be surprised, for example, to see a 5e Dark Sun iteration spawn a set of optional 'hard survival mode' optional rules which will then appear in some upcoming Everything book. Or horror/corruption rules spun off a 5e Ravenloft. Or even 'mercy mode' rules that cut down lethality and emphasise storytelling for something like a talking animal/fairy tale or kids-at-wizards-school setting.
I find it really bewildering to hear people saying that WotC have run out of subclass concepts though. Jeez, I could think of a dozen off the top of my head. And that's WITHOUT going into a very non-standard setting like Dark Sun or Planescape or Spelljammer which has its own set of archetypes, or going into game-mechanical places that WotC has thus far refused to tread, like subclasses that remove some of the base class features in exchange for better subclass features (a non-spellcasting implementation of the bard or ranger, for instance, or an unarmored cloistered cleric type), or subclasses that use things like hit dice or inspiration dice to fuel abilities.
