I do see what you're getting at, T and accept your logic. I'm just trying (somewhat unsuccessfully!) to both rationalise & weigh up all the alternatives that are open to me, financially and practically. It's complicated, as I've attempted to explain....
I do subscribe to 2 movie streaming services, Netflix & Amazon and 4K UHD content is available (at no extra premium) on the latter, which is great and I've been able to appreciate the leap in quality over my previous LG HD TV. I have quite a library of Blu-Ray & DVD titles, hence my thoughts about upgrading to a 4K UHD model which would comparatively upscale my existing discs and play any future 4K discs that I buy....probably pre-owned off eBay.
I realise that the 'sensible' route will be to invest in a more up-to-date AV receiver in order to fully enjoy the quality of sound that my new TV can produce, but as I've stated previously, there are several factors to consider. Even installing a centre speaker poses some problems. The most compact one (a Yamaha) that I've seen, would intrude a couple of inches above the bottom edge of the TV. Not ideal! I've really no other place to site it, so I'm still unable to reconcile the various limitations I'm faced with, with the most suitable route to better overall sound.