Obviously, dust settles on flat horizontal surfaces, so having an air intake at the very bottom isn't optimal there. And yes, if the Mac stands in an office with daily cleaning, there might not be much dust, but still, if you run it for 5+ years, it will suck in a lot. If it is in a corner of a small desk with lots of other stuff and not daily cleaning - e.g. at home, there can be considerably more dust.

My MB Pro started failing for thermal reasons after 5 years, fortunately, the local service partner could clean it for a tip. After that, the thermals of the machine were distinctively improved.

Dust in the ventilation is just a matter of time, a machine should not fail because the user can't clean that dust.