Fortunately people did do that at the time so we can see the results.
And the Spectrum had thousands of sophisticated 3d games running in real time and the C64... Didn't. It had the games it's sprite bitting hardware could support. People didn't even try the sophisticated software that was coming out on the Speccy.
...uhm... what were those "sophisticated 3D games" on the ZX Spectrum?
There were a lot of "isometric top-down camera" games with a static non-scrolling background, but those were hardly taxing, as the background was "slowly" rendered upfront and didn't need to update each frame.
And the Spectrum had thousands of sophisticated 3d games running in real time and the C64... Didn't. It had the games it's sprite bitting hardware could support. People didn't even try the sophisticated software that was coming out on the Speccy.