As critical as I am of D&D 5e, this doesn't tell us anything.
Any time there's a big change, there will always be a huge spike and then a falloff. That's just how things work. Notice, for example, that the recent low point is still ~1800, while the (presumptively) non-spike high point at the start is only ~2800. Ignoring the spike periods and trying to get a loose average, it looks like numbers have declined from around 2200-2300 down to around 2000. That's a drop, to be sure, on the order of 10%-15%...but if this is supposed to be "the sky is falling", I don't buy it.
Again, I say this as someone who is a HUGE critic of 5e. I very rarely have nice things to say about it, as most people on this subforum can attest. This isn't evidence of any kind of enormous backlash. If anything, it's merely representative of a slow decline that was already going before 5.5e launched.
And that, IMO, reflects pretty much what I was already expecting. 5.5e, like all "revised" editions, would provide a short-term bump. Updated re-releases never have the same staying power as the original, even if they're objectively superior to the original (and there's plenty of debate as to whether 5.5e is superior). Even when it isn't an actual revision, just a new entry point, you still see this phenomenon--both 2e and 4e never got an "X.5" revision, but they did get alternate entry points, and those only delayed things.
5.5e probably has another 3-5 years left in the tank. While the next year or so will probably be dedicated to adding new content (mostly to fill out the roster of options that were in 5.0 and didn't get immediately ported over), WotC is going to be at least considering 6th Edition from there on out.