>and if you are shooting still (and not video) there's really no good reason to ever delete an image off an SD card

There are tons of good reasons.

When downloading images off the card, software has to read all the files on it - which can take a very long time if the card is full of photos you've already processed in a previous session.

Then there's that you shouldn't be keeping most of the shots you take. Unless you're a still life artiste, most people (including professionals) take multiple pictures to account for blinking, moving objects, slightly different angles, etc. You should keep the best shots and delete the rest - storage is cheap but having to go back through all the garbage to find the good shots in the future is pointless.

Modern cameras have large sensors that produce large files. It's wasteful to keep buying more and more SSD cards. Just build a NAS or pay for cloud storage.