The most important thing about email is that you get your own, immutable copy of every message. You can keep your copy and know that it will be exactly the same when you look at it next week, it won't have been silently (or even not silently) "corrected".
It's both a blessing and a curse, as anybody who's accidentally emailed the wrong people or hit send before finishing editing knows, but it's the most powerful thing about email (and SMS or even fax for that matter!) - you get your own copy.
That is only one important aspect. It's also provider-independent (as long as you own the domain, that is true) and is, by nature, decentralised. (Like really decentralised, even by redundancy provaded by MX record priority in DNS.)
Unfortunately very few people have their own domain for email (although they should!).
When companies have kindly sent me their entire customer list (by using CC in place of BCC) I see there are very few domains in it outside of the major free email providers, and those that remain are mostly for their work or small business. A personal or family domain is very rare.
I have my own domain, and I really should be using me@myowndomain but I don't. I use gmail. One reason is that so many people and services already know the gmail one so it's hard to change, but the biggest reason is that I don't trust myself nor the place where I registered for my domain to actually maintain the domain name registry and email delivery for say 10 or 20 years without any maintenance.
The value of these behemoth services is that they are more or less guaranteed to be more stable than anything you can configure yourself. They are too big to fail. Even just pointing my me@myowndomain to gmail doesn't help here. It just adds a new weakest link to the chain.
I've had domains registered for over 20 years already, much longer than gmail has existed.
You can renew a domain for ten years in advance and easily transfer between registrars (and email providers, I use Fastmail), so it's a lot more reliable for the (very) long term than a third-party service.
You can also assume that copy will remain readable - standards and multiple vendors matter.
I have no huge issue with moving to a more IM-based interaction as my default, but I've yet to find one that is actually compatible with them all. I need 3-5 apps to communicate to everyone right now, and in a few years some of those will be gone and new ones will be added.
This is part of why I send email in plain text. HTML is probably not going away any time soon, but plain text has been around several decades longer at least.
At least HTML can be trivially converted to text. Who wants to bet that your conversations in WhatsApp, or WeChat, or Facebook, or whatever the kids are using today, are binary blobs that require a running, proprietary "app" to view, or only available through a cloud service?
I often look up E-mails from 5-10 years ago to find some piece of obscure information. It's a really nice archive for exchange of thoughts. I don't think my Slack or Facebook messages will stay around for that long.
It's both a blessing and a curse, as anybody who's accidentally emailed the wrong people or hit send before finishing editing knows, but it's the most powerful thing about email (and SMS or even fax for that matter!) - you get your own copy.