I think the situation is a bit different. While some companies have used XMPP for chat, I don't recall that any had advertised prominently the notion that you could use their chat system to talk to people on a different network. This was something that geeks could do (and did), but was never a selling point for the vast majority of users.
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.)
It is a sad sign of the times, you can no longer (easily) set up an email server of your own and expect emails to be delivered to everyone.
Managing O365 for a few domains I quite frequently see mail from not-so-small companies with their own servers getting stuck in MS Quarantine filtering.
My email server is running on a cheap dedicated machine, with SPF and DKIM (and SSL) correctly set up, and which is only used to send my own email (no mass mailing, etc.). In many years of using this as my email system, I recall only two cases where my email was rejected: by a university, which required manual whitelisting; and by GMail once for an incomprehensible reason. Of course greylisting (delaying for a few hours the first time) is frequent, but that's fine. So I wouldn't say the picture is as grim as you suggest.
Maybe there were more cases where my mail got lost because of excessive filtering on other providers and where I didn't notice it, but there comes a point where I don't care and I can just blame the recipient if they use a provider which silently ignores some incoming mail...
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.)