I'd be surprised if it was all that much more significant than the jump from 3.5->3.75/PF unless there is some kind of bomb that somehow reshapes ttrpgs & the people who play them or something. That doesn't mean minor changes & a bunch of well developed optional bolt on systems wouldn't make for a huge improvement to folks wanting styles of games different from whatever the OneTrueWay is
I would probably ditch D&D if it was half as much of a jump as 3 to 3.5. At least in terms of following the game and buying new products.
I’d abandoned D&D for most of 3.5 already, only coming back for 4e.
I doubt I’d be in the majority, but there’d be enough other people who lost interest that it would be quite a risk, especially any time before sales have stagnated and other strategies have failed to work.
I’ll be genuinely shocked if we see anything that can reasonably be called a new edition any time before 2030.
I’d say you’d need at least most of; a full year or more of stagnant or failing book
and digital sales, failure of other strategies to turn it around, a drop in licensed merch sales, failure of D&D shows and movies, and a plateau of other RPGs growth.
What we may see in that time, instead, is for licensed products to leverage the D&D brand to the point where brand stability is more important than book sales to the profitability of Wotc.
Most new gamers aren’t really going to care if there is one new book a year, when there are multiple video games, shows, movies, and whatever else.
The whole dynamic people imagine, where the point of a D&D movie is
at all to get more people playing D&D, is, IMO, not a thing.