I was trying to think how Plex thinks this is going to play out, knowing that this move will piss off their customer base. Then I realized, this isn't a play for Plex's existing customer base. This is a play for their customer's "friends and family" that are enjoying shared libraries already.
Their 'customer' base has for many many years been developing a large user base of technologically naive people with Plex apps installed who could never run their own server. If Plex knows, for example, that for every paying customer there's three other users pulling from someone's library, that's a huge opportunity for them to convert those users to paying customers.
Everyone that set up a Plex server and then shared it with your tech-phobic parents, cousins, friends, etc... We made this possible.
I don't like it but I can't argue with the logic from Plex here.