Part of that may be "due diligence." If the battery they sell you blows up because it can't handle the current, they can be held responsible.
Not selling because it's for vaping... well, most of the actual makers are clear that their products aren't for vaping, but it may be related to CA regulations. Or may be because they don't carry anything with a high enough CDR for our purposes. Although in that case, I think a "we don't carry anything suitable" would have been the response then.
While individual sources may dry up, lithium batteries are here to stay. (At least until something better is developed.)
All the products used for rebuilding will be ok. Rayon, cotton, resistance wire, PG and VG will survive the FDA. Same with flavorings, although they may not be able to sell them as "for vaping."
Nic, well, hard to say. Mods, tanks, coils may become black market items or seized by Customs, so good to have a healthy supply of those.
At the rate I'm going, I have enough replaceable battery mods for a couple lifetimes, same with RTAs (and repair parts), and 7 liters of 100 mg nic on ice.