As a software developer I want strong abstractions without bloat.
LLMs are so successful in part because they are a really strong abstraction. You feed in text and you get back text. Depending on the model and other parameters your results may be better or worse, but changing from eg. Claude to ChatGPT is as simple as swapping out one request with another.
If what I want is to run AI tasks, then GPUs are a poor abstraction. It's very complicated (as Fly have discovered) to share them securely. The amount of GPU you need could vary dramatically. You need to worry about drivers. You need to worry about all kinds of things. There is very little bloat to the ChatGPT-style abstraction, because the network overhead is a negligable part of the overall cost.
If I say I don't want magic, what I really mean is that I don't trust the strength of the abstraction that is being offered. For example, when a distributed SQL database claims to be PostgreSQL compatible, it might just mean it's wire compatible, so none of my existing queries will actually work. It might have all the same functions but be missing support for stored procedures. The transaction isolation might be a lie. It's not that these databases are bad, it's that "PostgreSQL as a whole" cannot serve as a strong abstraction boundary - the API surface is simply too large and complex, and too many implementation details are exposed.
It's the same reason people like containers: running your application on an existing system is a very poor abstraction. The API surface of a modern linux distro is huge, and includes everything from what libraries come pre-installed to the file-system layout. On the other hand the kernel API is (in comparison) small and stable, and so you can swap out either side without too much fear.
K8S can be a very good abstraction if you deploy a lot of services to multiple VMs and need a lot of control over how they are scaled up and down. If you're deploying a single container to a VM, it's massively bloated.
TLDR: Abstractions can be good and bad, both inherently, and depending on your use-case. Make the right choice based on your needs. Fly are probably correct that their GPU offering is a bad abstraction for many of their customer's needs.