A mostly good article, but to be honest, there are a number of things where I think you are wrong. One is: "From these, the email point seems to be most critical. In 2025, you really want a big provider to handle your emails - this point is so non-controversial that you have hosting companies advocating against hosting your own email."
Actually, letting a big provider handle my emails is absolutely the last thing I want - simply because, by design, they do it badly. If you want someone else to decide what you should and should not receive (in the name of blocking spam) - fine. If you want your incoming mail to be randomly tossed in the digital bin, without either the sender or yourself having any clue that's happened - fine. If you want to be subject to someone else's whims as to what's acceptable - fine. And as you point out in the blog, if you want to be subject to being disconnected "because ..." - fine If you want to be at the risk of your account (and all the email in it) to be deleted (whether by accident or malice) - fine. If you are fine with all of these then go ahead and trust your email to someone else and someone else's computer.
OK, I'm a techie and been in It for <cough> decades. But it actually isn't hard to run your own mail server. And if you do, then you can decide your own policies as to what is and isn't spam, and the biggest for me is that you can filter mail the only sane way (and which AFAIK not one of the big providers does) - and that is to decide whether it's going to be delivered, and if it isn't then you don't accept it. Lest there be any doubt there - AFAIK every large provider will accept mail (so to the sender's mail server "it's been delivered") and then throw some of it away.
Of course, someone with services to sell will egg up the problems. DNS is not hard. SPF is not hard. DMARC is not hard. Staying off blacklists is not hard (if you are only handling your own traffic, and generally not too much of a problem if doing it for clients). It is true that the big providers are more than happy to put blocks in the way of small providers (they'd rather you gave them money to mis-manage your email) - but in more than 2 decades of running my own mailserver (and with a previous work hat on, running one for clients) there have been relatively few issues. In general, the hardest part (running my own mail, on my own server at home) has been finding the forms the various providers have to say "actually, this is a real mailserver at this address" when blocked for being a "residential" address - but once found, I've found even AOL would accept my mail !
For good measure, I have access ot the unadulterated logs, and can turn up the logging level if needed to debug a problem.
I can understand hosting providers being against running your own mail servers. Firstly, again you usually are competing with a service they'd be happy to sell you. Secondly, I suspect that while it's not too onerous, there will be many clients who won't manage it and then expect the hsoting provider to sort out whatever mess they've got into. Other providers are simply against it because they think you should be paying them instead - but that's just business.
Lastly, there is one really REALLY important reason why any business (in Europe or the UK) cannot outsource it's email to the big providers - it is my opinion that it cannot be done legally !
I know that (for example) Microsoft will provide assurances that you can, you can arrange for all your data to stay within Eurpoean data centres, and so on. But you hinted that DNS is hard - and if you use Microsoft's mail systems, then the domain microsoft.com is involved. That is under US control, and so in the US they have the power to direct your login and authentication anywhere they want - e.g. they could collect an authentication token to give themselves access, or just plain route you through a dummy sign-in and harvest your credentials. Now, if someone has that level of control, can you tell me my data is 100% safe from them ? And if it isn't, then it's illegal under European and UK data protection laws to put any personal information (and that will include email) at that risk. Perhaps I'm just paranoid, but we've seen Microsoft hand over data held in data centre in Ireland as soon as the CLOUD act was passed.