Not long ago I took a short trip in a small Cessna (I think it was a 152). The audio quality in the headphones wasn't any better than what I hear on those ATC YouTube channels. I didn't understand a word of it (well, that's not quite true, I understood some words here and there; not nearly enough to be able to safely fly).
A Cessna 152 is not an airliner, obviously, but I would think audio quality shouldn't differ all that much between them.
The 152 could have had a 40 year old radio or a crappy antenna. Who knows. As you pointed out - that 152 is subject to a lot less maintenance scrutiny than commercial airliners are.
The other thing that plays in here is that ATC language is very highly standardized. In any specific phase of flight, the pilot will generally know what to expect next from ATC. This allows them to discern instructions (and read them back for verification) even if the signal is weak. In situations like these, ATC may seem completely unintelligible to people not used to ATC language and patterns.
A Cessna 152 is not an airliner, obviously, but I would think audio quality shouldn't differ all that much between them.