You seem to be conflating the choice; really, it's a choice of adding Google to the list of MITM or not. This system can't remove your mobile carrier from the circuit or stop them playing with your data.

This is not strictly true. In most cases, Chrome uses an encrypted (HTTP/2) connection to the Flywheel proxy which would bypass by ISP-side middleboxes. However, some carriers are downgrading the Flywheel connection to HTTP for the purposes of implementing adult content filtering; see https://support.google.com/chrome/answer/3517349. So we do "remove the mobile carrier from the circuit" in most cases.

Why do you trust an advertising company more than a telecom?

For me, it is because there is an expectation that my data will pass through the telecom. Not only is it expected, but as the end user I have little choice in the matter. If Eve wanted to inspect my traffic, that is a one-stop shop.

Flywheel ends up masquerading some of that traffic, so if for no other reason, it is atypical. I also perceive that it would be in Google's best interest not to abuse that privilege since advertisements are how they make their money. If they abuse that privilege, consumers will go elsewhere and they will lose their market advantage.

You only need but look at the cookie tracking the telecoms are doing right now to see that their oligopoly gives them little incentive to respect consumer's privacy.

> their oligopoly gives them little incentive to respect consumer's privacy

It's true they may not have much incentive to protect your privacy (besides perhaps competition from "better" companies and/or legislation).

But also keep in mind that Google has a huge incentive to breach your privacy, and have been taken to court over it, numerous times.