They could win privacy-conscious users back by offering an ad and tracking free Google experience with the paid plans.
For example, even if they don't need the data now because you're paying not to see ads, they might still collect it so that they can show you "better" ads if/when you stop paying.
However, now that we're where we are, it's just so easy to keep tracking you, since they'd actually have to go out of their way to remove it from their tech stack, that doing it on the off chance that either A: someday you'll stop being a paid customer or B: someday a PM will need to show more revenue and they decide to stick ads on their paid service anyhow, the combined probability of which approaches 100%, is worth doing it. Path dependencies can cause strange things to happen.
I can't trust a company who has tracking everything and everybody woven into their tech stack at every level to stop.
So the answer is that I would trust that it (mostly) doesn't happen if the legal terms say it won't.
With a good ToS in place, Google is actually more trustworthy than smaller companies.