Users get to use networks on terms dictated by their ISP’s. My ISP blocks self-hosted email. They did so because it was not in their interest – spammers were using the functionality to run spam ops. They still allow for self-hosting, but as self-hosting becomes more popular, ISPs’ residential networks are going to become a security minefield and an increasing liability. They will tighten the screws on what people are allowed to self-host and how, or they’ll just make it painful to impossible.
You could do a “self-hosted” turnkey email VPS, I guess, but then the users have to rent and spin up VPS’s. You could run a VPS provider that provides an API to streamline the process, but now you’re positioning yourself to be the next big cloud provider instead of decentralizing the web.
I wish I could just buy more RAM every time I hit a memory constraint.
EDIT: There’s a more general performance reason for using swap at the default settings (doesn’t cover every case but is fine for lots of situations). At the default settings it will start actively swapping at about 40% memory used. This is because the system actively benefits from the fs cache mentioned in the article and performance suffers in low-memory conditions due to the fs cache not having free RAM to work with. You’re waiting more on I/O (which has a big performance hit even with fast storage) as opposed to getting files from the cache. As RAM use increases, you can swap some of the less-needed program code to disk to keep more free space available for the disk cache. The default swappiness parameter might not be optimal for your computer/RAM use patterns and you might need to do some experimenting to find optimal values, but overall some amount of swapping is probably a good idea