If your average grandparent runs Windows Server, learning a new browser every now and then would be the least of their problems, IMHO.
The most important items are all pretty close to their old positions. Odds are she is going to Yahoo to search for Hotmail or Facebook anyway.
https://www.cvedetails.com/vulnerability-list/vendor_id-26/p...
I'd much rather Grandma have to learn to do something slightly differently than to hear about how Grandma had her bank account information stolen by some JavaScript malware.
For those with genuine use cases (I.e. ancient software that doesn't even work with the compatibility shims) I agree that there should be some way to run IE11 and older (and with only themselves to blame if they get pwned by ignoring the risks).
This attitude is responsible for more harm than any other single idea in software development.
Do not break your users.
You think you have a choice here but what really happens is Grandma stays on old software forever because the beatings have taught her to never update anything. And the really sad part is that she's right.
Grandma can get an old iPad for Hotmail and Facebook, which is far safer, far less needing of support and what most people are using for those types of things anyway.
Which is effectively what Edge does. It redirects all IE11 COM endpoints to WebView2, which is Chrome-based and should Work Just Fine for the foreseeable future.
I don't know what value of "grandma" is subject to being averaged, but this feels needlessly hyperbolic.
For the average Grandma who just wants to use Hotmail and Facebook, learning Edge is actually a monumental undertaking