And a lot depends on who is doing the looking. The real wake-up call is trying to get an image past a stock photo inspector. They inspect nearly every pixel for noise, artifacts, and any other up-res evidence.
In the real world I agree you can really do decent work with very small files, especially with landscapes. I enlarged a full frame image from a customer's small (5 mp) camera and blew it up to fill a queen-size bed headboard on Epson canvas. It helped that it was of a couple walking on a beach at sunset with very little detail. Any stock photo inspector would have rejected it in about two seconds and called other inspectors over to have a good laugh. My client absolutely loved it. So I guess it just depends.