> I bet you could understand almost everything with all vowels replaced with schwas.

No. Have you tried understanding someone who can only pronounce consonants but also whose dialect is foreign? It's unintelligible. Vowels absolutely serve a purpose. You're just used to hearing your own words spoken back to you in the same way you've always expected them.

Different accents use different vowels, but they remain comprehensible. It's specifically those accents that also change consonants that are taken as hard to understand, such as Scouse.

You can replace unstressed vowels with schwas and usually end up with something understandable. Stressed vowels cannot be schwaed. This is also specific to american English and doesn't work for other dialects, or even all american accents.