If I would play forum bingo, I'd put lazy design in the middle.
It is a sysnonym for: I don't like the way they are doing it.
Or: I rather have 234480 subsystems. Because having a single one is too noob friendly.
The core problem with this argument is
spells are not even remotely a simple system. They're by far the MOST complex thing in 5e, for like three or four different reasons (traditionalism, which is not at all the same thing as actually being traditional; spells being forced to cover every possible niche, no matter how inappropriate/distant from the ways spells work; and
actual traditional elements, like saving throws, slot scaling, spell components, etc.)
Having a single system is only actually valuable if that single system isn't ridiculously complex. It's like having a single, universal power plug converter--it has to have 17 different sockets and plug types because every country does things a little differently, making the converter actually really
complicated to use, despite being a one-stop-shop.
You can't just say, "One system is better than a quarter million!" Okay. Sure. There weren't a quarter million before, so the hyperbole does you rather a disservice. And it's not like we
don't have additional content anyway, like Superiority Dice and their many (allegedly) "supernatural" cousins like Psi Dice.
Maybe it is the case that having one, universal system really is better. Or maybe it's the case that having two complementary systems, that can each be individually simpler because they're tailored for their specific niche, would in fact actually be simpler than one universal system. Imagine trying to make one single universal language by just shoehorning the vocabulary of every world language into, say, Mandarin or English. It'd be nearly incomprehensible and INCREDIBLY difficult to learn, despite being "one system" that "everyone" could speak.
"Less is more" is only true if you do, in fact, DO MORE with less. I have yet to see good evidence that using spells for at least 90% of all supernatural effects, and progressively swallowing up ever more
non-spellcasting features into the spellcasting system, does in fact do more. At which point, all you have is "less is less." Which isn't exactly a compelling argument.