But the hardest part of hosting anything is the maintenance over time.
We tend to underappreciate the importance of time in everything. A button click can instantiate something powerful (and useful (and easy-to-use...)), but it will degrade over time, and eventually flat-out stop working.
I had a stack that worked just fine for my own needs, but it ran on shudder Python 2.7 -- everyone knows how that worked out (I chose to rebuild my stack on a different platform).
Software doesn't degrade over time (other than, you know things like cosmic ray bit flips, but in most realistic situations that should be fully mitigatable.)
The needs of the software user (including hardware and software they want the piece of software to interact with) may evolve, but that's different than software degrading over time.
> I had a stack that worked just fine for my own needs, but it ran on shudder Python 2.7 -- everyone knows how that worked out
While there's no further first party support for that version of Python, if it worked properly before, Python 2.7 and the software running on it probably still works properly now.
Ditto the average Rails/Python/Javascript project, as anyone who's tried to resurrect one that's gone so much as six months without being touched can attest. Which might not matter except that a ton of the software people might actually want to self-host are in one or more of those high-entropy ecosystems. Extraordinary levels of care and organization on the part of the creators and maintainers can mitigate this, but that amount of taste and effort is vanishingly rare.
These are degradation due to a changing environment, sure, but I wouldn't describe it as due to evolution in the needs of the user (presumably "must not have any well-publicized remote vulnerabilities" was a need from the beginning).
Otherwise, you are going to be influenced by external factors (security vulnerabilities, wanting to use a feature only available on a newer language version or OS, etc.) If you are a business, you'll also run into more practical concerns, like engineers not wanting to work on a mountain of technical debt.
“Degradation over time” was being cited as a reason not to self-host. Pointing out that not self-hosting exposes you to risk of others changing the environment so it no longer supports your software is a diametrically-opposed argument.