But what else can we do if we can't resell or return digital copies?

Buy the games when they are on sale or doing more research before buying a game. Thats pretty much all we can do.

Vote with your wallet first and foremost and voice your opinion somewhere where it will be heard, I'm guessing. The wallet thing is the thing publishers primarily care about, and voicing your opinion in addition to voting with your wallet will make it easier for publishers to see why you voted the way you did with your wallet.

To be clear, this might not be a very effective way, but it's the best I've got. I've voted with my wallet regarding DRM for a very long time, but it's not very effective, and you might miss out on games you care about or get to play them late. Alternatively you could try to start some kind of a movement to change the existing practices but reasonably, that's also probably not going to go well. Sometimes there are results, but most of the time when people complain or try to organize anything, it doesn't seem to be very successful.

I've been voting with my wallet more these days. I stopped buying Ubisoft games because of Ubisoft's own terrible actions, same with EA and Activision, and I even apply this to non-gaming media; I stopped buying Hasbro products because of allegations they're forcing Peppa Pig child actors to sign their voices over for AI use, and stopped buying from Amazon because of how shitty they are to their employees.

I guess. It's not too hard, considering how much internet access we have these days.

Very nice! It feels like a mostly thankless path in my experience, but at least you know you're doing your part.

I buy physical whenever possible, as an Xbox Series X owner my choices are sometimes limited though. Industry is going in a bad direction, I don't really see the downside with offering choice to gamers instead of forcing an all digital only future.

I try to encourage others to follow my lead, but I don't know if it's working.

And Sony is incredibly stupid for trying to force a digital only future onto their customers. Most people clearly don't want a digital only gaming market, because you can't trade in digital games, digital releases aren't guaranteed to be permanent, and neither are digital storefronts. Sure, Nintendo at least carried the Switch eShop and Switch Online over to the Switch 2, and Steam has been around for more than a decade, but there's still the risk of them disappearing.

Them, Steam, Epic, EA, etc. 

Good. Doubt Epic and EA would do that though. They're not known for having a sense of common decency.

Sadly, I feel like the majority wants this - or rather, doesn't really care whether physical stays or not. I'm not sure Sony would feel comfortable enough doing this otherwise.