A looser tie to definition (which is what I interpret your use of resolution as) is computation. Representing an 1080i stream in MPEG2 takes more bits per secon

A looser tie to definition (which is what I interpret your use of resolution as) is computation. Representing an 1080i stream in MPEG2 takes more bits per second than representing that same stream in MPEG4 or MPEG5. So if you keep the bandwidth constant, each generation has a rough maximum definition. MPEG 4 is great for HD, but MPEG 5 is probably what you want if you want to do UHD because the bandwidth won't increase. I don't negate the idea that a 10-bit field for the horizontal pixel count would be a significant barrier, but if it were super hard to come up with more efficient compression, some sort of extension would be defined to allow bigger horizontal pixel counts.