Plummet might be a bit strong, but there might be something to this.
There will be some draws to upgrade, but likely not as many as in the past. Large high resolution screens with good HDR capability exist now, and there are fewer newer features added to them than in the past.
I expect marketers to push 8K like it is the second coming of Christ, despite there being next to no content, and 4k already outresolving the eye at any screen size and viewing distance that fits inside the field of view of the human eye. Most people will probably fall for it too.
There will be some tech improvements that people want. In the near future people might be looking to add models with VRR and 120hz capability for gaming, if they haven't already. HDR seems like it is always improving as well. Certainly there will be fewer reasons to upgrade going forward, but that doesn't seem to stop the golden-eared audiophile types who keep dumping increasing sums into hardware that doesn't matter, so I'd imagine that will happen in TV screens too.
The smart apps built into TVs will suffer from designed obsolescence and get slow over time forcing the upgrade itch as well.
I think you are on to something here. This is why I always think external sources are much better than "Smart" TV's. If I had the choice, I'd buy top level screen quality and features, with no "Smart" technology built in, and use my own sources.
Personally I am still on my old 65" 1080p Panasonic Plasma I've had for about a decade. I do have the upgrade itch, and I almost bought a new 77" LG OLED a couple of months ago, but then I decided to pursue a home purchase instead, so I needed to save my cash for closing costs.
Once I do get the OLED, I picture holding on to it as long as I can. Since I use external sources, and I consider modern current gen HDR standards to be quite good, I have a difficult time envisioning any must hvae features that would force me to upgrade for another 10-15 years. The only limiting factor would likely be OLED burn-in/wear out where contrast is lost over time.