I would don't see how a sponsor could get rid of freedom of speech.
If they don't like what we say, they can just quit sponsoring.
That's why the car magazines are beholden to big automakers. Ford, GM, Toyota all buy back covers (and ostensibly, front covers) , two page inside spreads, content pages, even multiple pages with each selling a different product line or model. Even Tesla was demanding front cover status for test drives.
Look at how those Auto Magazine "reviews" are written. Never anything bad. At most the criticisms are "levels of good". If a writer has a car that breaks down, falls apart, performs poorly and says so or just says the car sucks, then an editor then rewrites the text. Then if what get published is still too harsh then they get a call from the automaker. You bet they do. Sometimes as you say Clients just stop advertising. (there is a famous story there) These magazines make all their money off ads. Your newsstand or subscription barely buys the ink.
The Truth About Cars, Top Gear and a few others speak their mind. That's why informed people like them.
We have already been boycotted by the very company this site is dedicated to simply by speaking our minds. The idea of being edited to continue a dedicated revenue stream is frightening.