I've seen my fair share of designers who came out of "5e only" who have been reinventing different kinds of familiar wheels when it comes to how classes are structured. So it may be safer to say that it's not so much a sacred cow as it is the presumed starting point or norm.
No. Subclasses come with a bunch of stuff that may or may not be what you want. A talent tree for, say, an animal companion is both straight forward and allows customization without making that pet your whole self.
What do these RPGs have in place of subclass customization?
Very little. That's the best part. You get effectively feats every once in a while in Dragonbane, and in Sahdowdark there are a small number random class features. OSE is B/X so... nothing.
To be fair, when I'm playing those games I'm expecting my character to receive at least some mechanical boons as diegetic rewards.
Yes, but that is the goal. You make a single decision for your 5e character lvl 1-20. What subclass then done.. 5e goes out of its way to remove all options. We talk about "less complexity" but fail to realize that "less complexity" is dull and boring. A blank piece of paper is less complex than a novel, but I like the novel more.
that has nothing to do with how convoluted they are, that's granularity.
convolution is not the same thing as complexity,
You need to define what you mean by convoluted then, because when I used it for subclasses I meant that they were a mess of stacked options that were a pain to work with and demanded more than they gave.
This is a potatoe potAto thing. People who think simple design is boring say complexity. Those who want simplicity use negative words like convoluted instead.
