Encryption makes signals intelligence more difficult, not impossible. It is and has always been a game of cat and mouse.

There is still a lot you can learn from analyzing encrypted signals. The metadata itself may be quite valuable, and any world power undoubtedly does cryptanalysis.

Wouldn't there also be a benefit from knowing, for example, that there's a bunch of encrypted radio chatter on a military frequency coming from a rural Montana town with a population of 500? Even without knowing anything that's being said, it might indicate a military presence in that location.