To me the 8051 is like the 'old radio tubes'. Looks nice if you see them, but you won't build a new radio with them, will you?

I did, a few months ago I finished a '51 project (a LED lighting controller) with a BOM total of $1.98. (4 ADCs one DAC PWM ,,, all inside the chip)

The '51 still have a life, albeit, probably, only in high volume thingies. Also SILabs just released a whole bunch of new '51s specifically for things like the above priced as low as $0.21. Will a '51 do for x? look at the SILabs "peripheral rich" offerings, a simple "just the processor" '51 is definitely 'old radio tubes'

Yes, for most I'd use an ARM, but do not ignore the '51 for smaller things

Erik