It does work, because the companies will realize that gmail no longer delivers their emails and that they need to change their behavior. Also for example AWS SES (Simple Email Service) will give you clear warnings if it detects that recipients mark their email as spam (it seems that for example gmail delivers this information somehow to SES).

Few companies are stupid enough to use the same sender or even domain for marketing and important transactional mail.

Utility companies are unfortunately exactly that kind of stupid though.

My utility doesn't seem to market to me. What the heck are they sending email to you for? "Use more electricity!" "Build a new house!" "Get a hot tub?"

My gas company has the equivalent of a mileage program. You earn points per cubic meter of gas you consume, that can then be redeemed for expensive meals at restaurants and stuff.

Yes it's very stupid.

My telco does as well, and I got a free Nintendo Switch from them for just having fiber internet that I would need anyway (the telco just owns the fiber, the ISP then goes over that open fiber, so I pay two different companies, it's the former that has the point program despite being the definition of a dumb pipe)

I'm not sure if it would be called "marketing" but my power company would send "you're using more energy than your neighbors" (I worked from home) and "think about why you use the most energy at night" (in the winter) emails which were no better than spam.

Also ‘free energy audit!’, ‘sign up for our peak-load-and-we’ll shut off your AC program’, and ‘we’re good people, honest!’ promotions.