There’s this pattern: “everything should be done in small easy to understand units, which can easily be combined to form larger more complex systems”. Examples include microkernels, microservices, small packages, and small functions.
Sounds great, but applied everywhere will hit a snag: combining all those small easy-to-understand units to do useful real-world stuff is rather hard. There are plenty of research microkernel systems but general purpose microkernel systems are rare: almost all mainstream kernels are either monolithic (Linux, FreeBSD, OpenBSD) or hybrid (macOS, Windows), with just a few exceptions which are special-purpose (QNX), historical (AmigaOS), or incomplete (GNU Hurd).
Microservices seem awfully similar to microkernels. Communication between the different services is quite a hard to get right, and don’t even mention the orchestration of all those services. See: MicroservicePremium. In many ways it’s even harder than microkernels. Most “microservices” are really just using a hybrid approach which is perfectly reasonable, but underscores that “true” microservices are hard.
Software split in to small functions is often harder to understand. Sure, the individual functions may be easy to understand, but understanding the overal system is much harder.
Sometimes longer functions in the form of “do this, then do that, then do such” are okay, especially for more complex workflows that naturally belong together. Splitting this up in to tiny functions often doesn’t make the overall logic easier to understand.
A program should be understandable, not “small” or “DRY”; those are merely tools to achieve this understandability, but not end-goals in themselves. Applying any tool indiscriminately is not going to end well.
Indiscriminately following Test-Driven Development can be really harmful. I expanded on this at some length in my testing isn’t everything article, so I won’t repeat it here.
Global variables can be perfectly fine, especially if the program is small. Sometimes it just makes more sense. The downsides come at scale: when you can no longer see all global variables.
I suspect this is why function programming hasn’t taken over yet. In spite of all the advantages, actually making useful programs with it is harder than just writing a many simple functions.
As Haldane already observed many years ago, the “right” size isn’t the smallest (or the largest). It’s the right one.