Yeah that's how editions work Oofta. You know this.
There's no edition that wasn't true of. That includes 4E and Essentials. All the 3.5E changes could be summarized as "minor additions and adjustments". That doesn't mean D&D couldn't benefit from some larger changes, like with 1E going into 2E. It just means WotC have the basic common sense to hold off on those until a 6E.
Personally I think that as long as the put out what I'd called a 6E and what others call 5.5E and what yet others still will hilariously try to pretend isn't an edition change ("I see no ships"), sometime in the next five years (probably in 2024), they will continue to see double-digit growth year-on-year.
If they don't, I think that we'll see a re-focusing away from selling D&D as an RPG, more towards selling D&D as an IP/lifestyle product. It'll no longer be about actually playing D&D, or even them providing materials to support D&D, as the brand and the idea of being a "D&D player" (even if you aren't in a literal sense). This is also likely to provide decent year-on-year growth, until it hits a generation gap and is seen as hilariously uncool (which seems unlikely to happen with TT RPGs in general, given they've survived several such). Betting on it is hard, because the first time WotC drops significantly below 33% year-on-year growth, like, to say, below 20% or 25%, they'll just not mention the figure and refer instead to "great year on year growth" or something. This is how companies work with their PR. Figures until they're not favourable or as favourable - we saw this with Blizzard and WoW's growth for example, and we see similar with virtually all big MMOs.
Note too that WotC are already attempting to diversify and move into video games, in case D&D stops working out. If they do update D&D suspect we may see a non-D&D RPG to test the waters there too, but they're going to be a lot more cautious than the '00s WotC on that. If they go lifestyle brand they probably won't bother with any other RPGs, but will come out with more products to market the IP to a broad audience.
I think you're missing the point re: Dark Gifts etc. - they're just a sign that a lot of people have a different attitude to the game. I guarantee that if they do a 5.5/6E, the default situation will be that all PCs start with either a Boon of that kind or a Feat, if there's even a difference at that point. There will be an optional approach called like "Old school PCs" or something where they don't get it.