Not in core please ....

You can easiliy add it to endpage text actually.

Why not in core? Might a plugin be an option?
Define 'easily' :P (it means googling for html code that might work (or not), which is not particularly noob-friendly)

https://developers.facebook.com/docs/plugins/share-button/
https://about.twitter.com/fr/resources/buttons#tweet
etc ....

After : copy/paste on your endpage.pstpl

You expect, let's say, a philosophy student with no technical knowledge whatsoever at all to 1) find the page you're referring to, 2) go through the process in each of these websites and 3) figuring out how to make a similar button for email.

I appreciate you posting these links, but I guess my point is: sure I am capable, but many other students probably aren't (or put differently: they find the threshold to try too high) :)

Then plugin devlopment.