If you're flying to DL hubs or doing one-stop trips today, not much will change except for the connecting points and terminals. If you're used to flying nonstop to secondary airports (or even just non-DL hubs), you'll be in for a disappointment, as there is zero domestic service to places that are not Delta hubs or focus cities from SFO.

Internationally, you'll be connecting thru AMS via KLM or CDG via AF in Europe. Both of these are great options IMO, and you shouldn't have a huge problem with the frequency from those two airports, as well as the reach into Europe from those two hubs.

Personally, I love the DL setup at SFO - the SkyClub is fantastic, I've never waited a long time at security (and they have CLEAR + Pre Check), and everything is a fairly short walk. Before relocating to Seattle, I was doing JFK-SFO-vv at least monthly and found the experience to be nice on both ends.

Honestly, I'm curious if moving to UA doesn't make more sense. If I was SFO based, it's almost certainly what I'd be doing.