I imagined this to be the case when purchasing my $500 solar G-Shock, but it needed the battery replaced every 3-5 years, and the work had to be done by an authorized Casio repair center; local watch repair people refused it.
So, if we got a watch that didn't have a standard back, we just wouldn't bother with it. However, for the G-Shock, it looks like it's a really straightforward job that should take you 5 mins to do yourself. Looks a lot easier than any laptop repair/replacement job for example: https://www.youtube.com/watch?v=hGkFYVpDv-k
Anecdotally - I've had my solar powered Edifice watch for 10 years now and the battery still works fine.
It's a liability issue, not a capability issue.
If it really is a liability issue, then it's solved the same as with literally anything else in the world - you sign a little piece of paper that releases them of liability. Done. When I bring my own brake pads to fit to my car at a nearby garage they just make me sign a paper that they don't and won't accept any responsibility for fitting my own pads. Problem solved.
I went to a Macy's watch repair department, a well-known jewelry store, and two small watch repair shops (the "old man in a tiny office in an office building" type) highly ranked on Yelp. All refused.
Did they say why? I'm not surprised the large store did, but the "old man in a tiny office" type shouldn't have - why did they refuse? The only reason I can think of is if the battery is soldered on, but otherwise how is it different than replacing any other type of battery. I know my Edifice watch uses a regular button-type battery, it's just rechargable, but it's not soldered on or anything.
If I recall correctly, they said that they could not remove the case back and guarantee its waterproof integrity.