Compare this to email, where every user expects their email client to be able to email people on different providers. If, say, GMail suddenly stopped being usable to email people without a GMail address, all users would think it was broken, not just geeks.

The trend to centralize email is a more subtle one. For instance, make email work better when emailing someone at the same domain (also because it's technically easier to handle): I have seen people trained by GMail to send huge files as attachments because it works when sending them to other GMail users. Also, spam filtering: a small email provider sending email to a large one has a higher chance of being flagged as a spammer, and has to support all requisite technologies (SPF, DKIM, SSL if you don't want the email to be marked as insecure, not being on a blacklist or residential IP, etc.).

Long sorry short, I'd have trouble imagining that a major email provider could realistically stop supporting "federation". Maybe it could happen if major providers unite to create a walled garden between them and to cut off everyone else (unrealistic), or if one provider's market share increased to near-monopoly (somewhat unlikely for now because of the long tail of institutions, companies, universities, who still want to run their own). I think the main threat to email is non-federated systems that are more convenient to use on mobile (e.g., WhatsApp and others), or social networks in general. (This may sound far-fetched, but in France I do see many companies who advertise support on Twitter and Facebook but cannot be reached by email.)

A better question might be: what can possibly replace email as an online identity - and also - do such a better job at it that I feel inclined to switch the 100's of services over to it - a task I would not take lightly.

Email is so much more than just a protocol.