You care because Pierre de Fermat 16th century work on modular arithmetic was of “no intest to anyone else” except other theorists but it got to shine in the twentienth century in modern cryptography.

Doing theory is what gives you a chance to push the envelope to places you can’t even imagine when you’re knee deep in real world problems. And proving something is possible (if impractical) gives an incentive to find a better algorithm and may attract others.

For example see the first graph here: http://klotza.blogspot.com/2016/06/the-fall-of-twin-prime-ga...

Considering this line:

" In 1973, Jacques Morgenstern proved that you couldn’t get a Fourier transform faster than ½ n log n if you required that complex coefficients used in the computation had bounded magnitude"

I highly doubt this is going to somehow inspire people to make faster algorithms than the mathematical bound...

I have no problem with pure mathematics, my training was in it; but it seems very possible to me that most people funding theoretical computer science (as opposed to pure maths) probably assume the work has a more realistic computational model backing it up.

No, I don’t care. Until there is a real, practical result, I don’t care. There are plenty of examples of math discovered in service of solving a problem. There is nothing that says math has to be purely abstract first.