Well, first of all, keeping lithium batteries at 100% is hard on them, to the point where some devices lie to you about 100%. So staying at 75% and then topping up when it's cheap might not be the worst thing. The big deal would be is that you'd have to have a way to communicate with your car that you're going on a road trip Saturday 7:30 am/ in three hours and the battery better be all the way full by then or else.

I saw some pundit conversations about Battery Day recently. There's some speculation that a 'million mile battery' may be coming due to union of a few different complementary improvements to the battery design and manufacture.

If your battery can survive four times as many discharge cycles as the drive train, hooking back into the grid isn't so bad as long as your car doesn't conk out 10 miles shy of Grandma's house.