But would Grandma really want a phone like this?

If the original (non-Fire) Kindle is any indication; no. I gave mine a pre-Paperwhite Kindle and she loved it. Then someone bought her a Kindle Fire ("If you like that you'll LOVE this. You can play games!"). She hated it and went back to the regular Kindle.

Turns out, if she wants to play games she'll grab a deck of cards.

You're questioning whether someone wants a phone without easily available support? I'd venture to say if anyone marketed a phone with a 'feature' like that, it wouldn't do well.

Or are you questioning whether a grandmother want's a smart phone? The answer is yes.

Unfortunately, there are grandma's who are practically forced to have a smartphone "since the family plan has data" and...they would probably like live support (I'm just saying they do exist).

Wonder how large the cross-section is of people that could use that feature versus people that could remember / figure-out how to initiate that feature.

or just design it to be easy to use, and teach Grandma how to use it, and then she has time to bake for you