I actually don't use Gmail for partly the reasons you mention, so I'm not at all unsympathetic to your view!
However, I tend to think of governmental privacy threats as the most important ones, and I assume that many other people in this discussion (and in the community of people who strongly advocate decentralization) share that perspective. Your prioritization is sensible if you're much more concerned with corporate monitoring (especially if you live in a jurisdiction with effective data protection legislation).
You might also want to consider people who live under extremely violent or repressive governments, whose concern with government as a threat actor may be easier to sympathize with. They're probably better off in the short term with something like Google because Google will likely resist data requests from their governments, while the ISPs (being local and sometimes state-owned) will likely not resist those requests. While they can't resist every surveillance or coercion scenario, it's easy to imagine that they can fly under the radar easily by using Gmail if it's popular where they live and they aren't doing other things online that the government finds interesting.