I don't think it ever makes sense to switch to electric radiators. It might make sense to switch to a heatpump, but you need to avoid being hugely overcharged by the installers and there are flow issues if you have 8mm piping.

This ROI calculator looks reasonable: https://ukcalculator.com/solar-panel-roi-calculator.html - note that it subtracts the install cost for you, so any case where the final figure is positive is profitable. But of course that depends on whether grid prices go up or down in the next decade ...

Well France did move to electric everything (cooking, hot water, heating) in the past thanks to ample nuclear electricity production (and possibly not too harsh winters). Unfortunately they let their nuclear programme decay and are struggling now (EU grid integration does not help)...

I think a big part of the push to install heat pumps now is that it is understood that electricity production is in dire straights, taking into account that the transition to EV requires a lot of electricity.