Do you think that is a good situation?

Yes, it should be the most important aspect of it. If the devs can live with it through donations and the project becomes their full time work, good, but that should never come at the expense of the user.

Why "should" it be the goal of the dev? Who are you to decide that for the developer?

That’s just my vision of it. Everyone is free to do whatever they want, but for me that’s a requirement for FOSS projects.

That's a prediction and I don't know what data you're basing that off of. Could you share it?

Just a logical enshitification way. Profits always comes at a price. Keeping your project free while being for-profit often means getting forked and dying, much like ownCloud.

Those are all assumptions. You do not know if the search engine is making enough money already. They might be trying to make money in the first place and getting it in front of users might be a way to raise awareness popularity. There are also companies like ecosia, duckduckgo, qwant, startpage and others that do care about privacy. Would you be against their sponsorship too?

Whatever the end goal is, it’s still advertising and it’s impacting users' freedom of choice by setting a default and virtually discouraging the use of other engines.

This kind of info is often hidden (didn’t try Waterfox but I bet that it won’t say that the default search engine gives them money when you first start the browser), because they know it might make them look bad.