My spectrum was quickly forgotten as soon as I got my hands on an Amiga 500, so I'm not one of the people you're talking about.

But I suspect by concentrating on graphics quality and games you're missing where the nostalgia comes from.

The spectrum was fun to learn to program on in a way eg the C64 wasn't. It had a tangible personality even though it was cheap and a bit shit. Whenever I hear someone reminiscing about their spectrum, it usually isn't about the graphics or games but about it being their entry point into programming and how that lead to a career. It had a user friendly BASIC dialect (unlike the C64) and came with good programming manuals.

Nobody I knew with a C64 (or people I later heard about growing up with a C64) programmed it. It was just a games machine to them.