On reflection, I think what has happened, is that some internet things, because the internet is still relatively new, compared to many things in life. Have started out free, so YouTube has been free for a very long time.
But really costs a lots of money to run, needing lots of computers/servers, man power, premises, taxes and other bills to pay, big electricity bills, and all sorts of other costs.
They were getting some money, from advertisers, but some sources seem to say that, that has been declining, for various reasons, over the years.
So just like many other things in life, such as electricity bills, Petrol/Gas for cars, etc. It needs paying for.
Not to mention, some of the YouTube producers, also want to earn money from the videos.
So I can't really insist or suggest, that it remains entirely free for ever.