Diminishing returns?

Sure, there is a point where you will not get the best out of a speaker but most folks cannot hear electronics as well as they think they can. A pair of $5000 stereo mains would sound great. They'd sound better on better electronics but it's a stretch for many to hear it.

And, it would be helpful to better understand what 'expensive' means in regards to a speaker budget, low end, and high end. Five speakers (including the two mains) and subs or more?

Other considerations:  Primary use of system, TV or two-channel audio? Hopefully two (or more) good (not expensive) subwoofers as well.

I love movies but I'm an audio guy first. In your case I'd get better mains, a fair center and smaller surrounds plus two good inexpensive subs. I have the Elac Debut F5.2, center and B5.2 bookshelf surrounds for the smaller TV room (and an old REl sub). Elac is great for the money and there are a lot of other good economical mfrs these days, Golder Ear, PSB, etc.

soix is correct about being able to hear mass-market receivers vs good audio gear. My bad. I was thinking of all the higher end audio comparisons I've done over the years. That can be work to differentiate above certain levels of good quality. SVS is a good call on speakers as well as subs.

A lot of other interesting and fun suggestions. Since this system appears to need to do double duty, and since the picture implies two people watching, it means two people hearing, so each gets a center image that is either far left or far right depending on seating. Does not sound like fun to me.  (Of course, the OP and sig O could sit audio style, one in front of the other in wood chairs...)  Losing your HT center image is my gripe with telling them to ditch HT.

Ah yes, two-channel 'enthusiasts'. I admit to being one totally, but I still thought to suggested a center and surround solution (with better mains for two-ch use). It seemed to be what was needed per the O.P. unless I misunderstood. Sometimes it's not about what we want I guess.

I remember the days having to work it out for stereo and video in one system (adding TV sound in the 80s to a receiver). It was more challenging as the system grew and eventually separated.  These days I feel very lucky to have individual rooms/setups for high end two-channel audio, regular TV, and a home theater. It just takes time and if the O.P. builds carefully those items are very usable later. (plus I probably never sold a piece of audio gear in my life...)