I'm especially interested in the opinions of people who have recently paid the extortion fee. Will you be upgrading your subscription when they introduce YouTube Ultra with some small quality of life features, or will you wait until they start showing you ads again?
You've already proven to them that you're willing to cave. They have no incentive to keep the price of your subscription at its current level, nor to keep it free of ads.
Not only that, but they have more of an incentive to bump the price once they've extracted the most value out of their captive audience as they could. At a certain point they'll flatline against the more strong willed of us who will never collapse, and they'll just give up on us and start squeezing the people who already demonstrated a willingness to pay whatever Google demands.
So by subscribing to Premium, you've contributed a little bit toward that maximum, and done your part toward bringing the introduction of YouTube Ultra even closer...
But to answer your question - I don't think YouTube will add ads to current-day YouTube Premium. The amount of money YouTube makes by me paying the $7/m is much more than the money they make by selling ads to me (we know this because many creators have themselves noted that the average YT Premium subscriber makes them more money (CPM)). It is also trivially easy for YouTube to increase fees by $1/month without (much) backlash, which would probably still make YouTube more money than showing me a few more ads.
I would not be surprised if they add an ad-lite "YouTube Silver", but I don't think that's the direction Google wants to go in; they'd rather you pay more for more things. You can't get YT Premium without YT Music right now (the price of Music is baked in), and Google pushes Google One on me a lot.