There's also the long-time guarantee; if you write an application that runs on Ubuntu Server or Windows Server now, you can bet your ass that it will still run, unchanged, for another 10 years. The only maintenance you need to do is to fix your own bugs and maybe help out with some database stuff. If you deploy a Lambda app now, you have nothing to guarantee compatibility for such a long time other than "Amazon probably won't change their API, I think".
Using lambdas tie you to AWS because as soon as you use a few step functions, or you have a few lambdas interacting, changing to Azure or GCP becomes a huge pile of dev work and QA.
Having everything "just run" on linux instances let you be perfectly portable and now you can actually shop around.
Had you deployed an application to a proprietary cloud platform ten years ago, a handful of those services would have had their APIs changed or even been sunset by now.
I agree that it almost never happens, and that's why I run Debian as well. However, if you run production then things happen.
I've in this area professionally for some time now, and I've never "maintained" servers in any reasonable sense. There are kernel people who maintain the kernel and there are Debian devs who maintain the operating system. The server may be mine (but more often that not, it isn't) but only in very specific circumstances do I ever concern myself with maintaining any part of this stack.
A vanilla Linux VM is a platform to build on. Just like AWS or anything else. It is the environment in which my software runs.
Thus far, something like Debian has been more stable and much less of a moving target than any proprietary platform has been, cloud or non-cloud. Should a client wish to minimize maintenance costs of the software over the coming decade, it is most cost effective to not depend on specialized, proprietary, platform components.
That may change in the future, but right now there is no indication that is the case.