I'll take a shot here. I'm not sure why you think Gmail or the other biggies can actually guarantee delivery any better or worse than anyone else. In a technical sense, they definitely can't.

In an anecdotal sense, I mean, I've dealt with my hostdime stuff, gmail and outlook. My stuff and gmail deliver just fine. Outlook does fail.

In a practical sense, see issue above. I think "non-delivery" via the big-boys getting things wrong on spam (and worse future outcomes, pretty easy to imagine gmail trying to "amp" or something similar email such that it cuts out little guys) is a far far greater danger or problem than an individual or smaller outfit tinkering with their own servers and occasionally getting things wrong.