Your argument is fully valid and defederating might be the right thing to do for you.
In the end, I have to weigh their cost to scrape my data against my cost to access content that I need. As someone who has built scrapers for scientific purposes before, I can tell you that building something that scrapes Mastodon and Lemmy instances is not a single cent more expensive than getting my data through federation. It’s also probably a lot more reliable because they can get everything, not just what their users subscribe to. On the other hand, my cost for accessing my friends’ and family’s posts as well as corporate social media accounts if I don’t do it through federation is creating an account in their proprietary app. And then they will be able to get a lot more of my data than they could ever scrape from my Mastodon profile.