The original answer to "why does FastMail use their own hardware" is that when I started the company in 1999 there weren't many options. I actually originally used a single bare metal server at Rackspace, which at that time was a small scrappy startup. IIRC it cost $70/month. There weren't really practical VPS or SaaS alternatives back then for what I needed. Rob (the author of the linked article) joined a few months…
We had some old Compaq (?) servers, most of the newer stuff was Dell. Mix of windows and Linux servers.
Even with the Dell boxes, things wasn't really standard across different server generations, and every upgrade was bespoke, except in cases when we bought multiple boxes for redundancy/scaling of a particular service.
What I'd like to see is something like oxide computer servers that scales way down at least down to quarter rack. Like some kind of Supermicro meets backlblaze storage pod - but riffing on Joyent's idea of colocating storage and compute. A sort of composable mainframe for small businesses in the 2020s.
I guess maybe that is part of what Triton is all about.
But anyway - somewhere to start, and grow into the future with sensible redundancies and open source bios/firmware/etc.
Not typical situation for today, where you buy two (for redundancy) "big enough" boxes - and then need to reinvent your setup/deployment when you need two bigger boxes in three years.