Also, ads are very much “give an inch, take a mile” in nature. Once you open that door and have say a tiny banner ad, then it’s like “well that didn’t do much harm, let’s do a slightly bigger one”, then before you know it it you have sponsored posts, sidebar ads, videos and all that nonsense.

It’s just one of the avenues where enshittification creeps in IMO.

I see two misconceptions here.

First off, there is such a thing as reasonable ads, namely, ones you want to see. Sometimes you are looking to purchase something or some service, and you have to go out of your way to look for options. Ads for what you want, when you want is ideal. However, the point where advertising goes to far is trying to solve this problem by learning everything about you in order to effectively read your mind. That’s where it becomes unreasonable for me.

Second, the philosophy of lemmy (and federation in general) was not to tell all instances and users of said instances how they should use the internet, it was to give them the freedom to use the internet however they want, without sacrificing the connection with others who want to use it differently. It would not be equivalent to using reddit, because an instance that serves ads would have to compete with other instances who don’t. Whereas reddit is a walled garden where you are forced to take their ads along with their content, if a lemmy instance with ads became too “unreasonable”, the users have nothing to lose by leaving, the content is all federated.

The reality is, it’s not a matter if, but when we will see instances who try to fund themselves using ads. They’re free to serve them and you’re free to defederate from them in protest. That is the lemmy philosophy.

It’s unlikely ads would bring in any significant amount of money from them se people, in that case.

then it’s not really donating

No, this is rewarding the implementation of ads and can’t be thought of as donating anymore. You are paying to remove ads thus giving them money for having added ads to the site.