Alternate means of getting where they need to go is, as others have noted, KEY in the conversation.
My in-laws lived in Florida. MIL had not driven for years (she still had her license, but after a serious illness she just was too nervous to). FIL did, and was frankly scary. He’d had two at-fault accidents during the last few months of their car lease.
As the family was paying for that expense, we all agreed NO MORE CAR. FIL kept his license just in case but never used it; he was going to, when they wanted to evacuate from a hurricane, but we handled it by my flying down there and doing the driving. The car went bye-bye when FIL was 82 or so, I think.
Their community had a bus that went to certain local destinations (grocery, WalMart etc.) every day, and their county had a senior transit service that was… not always as reliable as it ought to be, but between that, and the occasional taxi, they were covered. Grocery delivery service was a huge help also. There was no family anywhere near, so they had to rely on those various services. Note that after-hours transportation availability should also be a consideration; they had trouble finding a cab to get them home after an ER visit at 11 at night (and I was similarly stranded at an ER for several hours, in another large city).
MIL was actually happy to be rid of the car (or so she said, frequently). FIL didn’t say that, and I think was rather disgruntled, but they didn’t have the means to pay for one themselves, so he really had no choice.
Where my daughter lives: if you can’t drive yourself, you’re screwed; virtually no taxi service available (which led to the fun experience of my having to walk a mile at midnight, to get to my hotel from the train station).
As far as having the actual conversation with the parents, I don’t have a lot to offer, beyond what I’ve heard others suggest over the years - things like talking to the parents’ doctor and so on. Good luck!
We looked at the Go Go Grandparent service at one point a couple years back, and wound up not pursuing it for a number of reasons (there’s a stiff surcharge, and there were some complaints about reliability). Plus if you are in an area where Lyft / Uber are scarce, it wouldn’t work (e.g. my daughter’s town).