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.
We will be squeezed step by step. Sad but it's going to happen.
I am still not premium subscriber, but something weird has been happening to me, whenever I click on want to watch older videos like 6-7 yrs old, it loads slower, also on such type of video, I always get grammarly ads. If I guess correctly it happens for around 20 sec then I see skip button.
I'm still using chrome browser.
Which is fine, if you prefer YouTube free, supported by ads then you're of course free to carry on with that. I'm sure Google are very happy to keep feeding you ads.
I'm curious then why you care about YouTube premium subscribers? Again your hypothetical question (around possible future Google actions) seems loaded. Almost as if you predict bad Google behaviour,then treat them with distain for that behaviour prematurely.
Of course you also have the option to stop watching YouTube completely. Then you'll be neither a premium, nor ad-revenue source for Google. That would really stick it to them.
The next boogeyman will be stockholders who want increased value on a product with a mature market segment.