It’s not about paying to not see ads. Anyone with an adblocker could understand that much. The point is to support an app you personally like, and appreciating what comes with an app that has an income stream.
Part of that comes with understanding that all things come at a cost. Like many have said above, FOSS comes at the expense of the time and money of the developers of the app themselves, and some of that cost is passed down to the consumer (anyone who uses FOSS without contributing to development is a consumer in the end, after all).
The consumer has to bear the cost of slower, more infrequent updates that are entirely dictated by the developers schedule and whim, with less focus or effort put into the design or other features. And honestly, if a consumer can’t tolerate that, that’s totally fine, that’s what dedicated teams and people who do these things for a job are for.
If you’re one of those people who doesn’t mind slower, less intuitive, or buggier software, then go ahead. But until you can actually prove that a FOSS offers better services than a marketable service, people are really just going to dismiss you as someone who can’t think for themselves.