Is it a problem that my parents' CA drivers licenses have expired? They don't drive, online renewal, even for an ID card isn't an option.

They won’t need valid drivers licenses but there may be times where they need a valid ID & some entities won’t accept an expired drivers license. If they can’t go to the DMV to get IDs, you can call the DMV & arrange for a field agent to come out to their house.

As long as they do not drive they do not need a valid license.
If you can get them to the DMV a State issued ID is a good thing to have but not necessary. They can keep the expired drivers license and that could serve as an ID in some cases BUT doctor offices, hospitals want to scan a valid ID or license along with insurance information without that they might not be able to process the claim.
Often the DMV will set up at a senior center and seniors can come in and get ID's with not much of a line.
Here in Illinois they now have a separate line for seniors so you do not have to wait in the main line so the process is a bit faster. Might want to check and see if CA does the same thing.

Ricky, are you thinking it's a problem because they won't have a state ID anymore? Both my very elderly aunts in FL (neither drives, one has dementia) have state IDs but I do believe they have to go in person to apply. I would imagine they should have IDs, even if they aren't very active or if they dementia. Probably their PoA will need to accompany them for this task. If they're not mobile you will need to call the DMV to find out how to get them or if they really need it.