Likely all existing cryptography would become crackable, possibly some of it, very readily.

(Assuming you mean P==NP)

Would it become crackable, or just theoretically crackable?

E.g. it's one thing to show it's possible to fly to Mars, it's another thing to actually do it.

Not really:

* It's possible - very likely even - that even if somehow P=NP, the fastest algorithm for any NP problem turns out to be something like n^1000, which is technically P, but not practical in any way.

* The proof may not be constructive, so we may just know that P=NP but it won't help us actually create an algorithm in P (nitpick: technically if P=NP there's a construction to create an algorithm that solves any NP problem in P time, but it's extremely slow - for example it involves iterating over all possible programs).

I think you read it backwards - that's a possible consequence of P==NP, not P!=NP.

Isn’t it the opposite?