A MITM doesn't require decrypting. It just requires telling a convincing enough lie to each side of a conversation that they think they're talking to the other party. Then you end up encrypting to their key which they can just read. That's different from breaking the encryption.

Are you sure they can’t decrypt?

No, and none of the state actors who do that kind of stuff will ever tell you. Watch The Imitation Game for a good/bad time and some context.

If you had a break for the modern crypto schemes and published it right now you would shut down all digital commerce, a lot of the spy traffic if not most of it^1^, and nothing could be started back up again until everything was redone from scratch. Best hope would be that the post quantum algorithms^2^ the cryptographers have aren't succeptible to whatever you did to break the existing algorithms. It'd take a bit to get the software implementations and keys in place, etc. The hardware stuff would take longer.

• [1] The stuff using One Time Pads would still be fine, but its limitations wouldn't take the place all of all the rest of it.
• [2] The rollout of post quantum algorithms is going on now on a more accelerated schedule than originally planned.