I lean on the same opinion, and it's not like that at all, if Liftoff or whichever application were to be on the store with a pricetag they would probably even be happy to pay for it, this is not about cost for us users, it's about being sold to trackers and advertisers and it being proprietary. You can argue that if the developer made it open source then someone would have just forked it off without all the restrictions and privacy problems and that version would have effectively taken the place of the original, that may or may not be true, it seems like a lot of people are faithful to the original developer.
If it were to happen though, I think it would speak numbers on the practice itself, this is fundamentally an ethical issue, the developer made an application that he maintained over the years, I have huge respect for that in and of itself, he also made it paid for, absolutely fine still, but the free version has ads, that's not cool, not because ads are bad, but because the current ad landscape depends on tracking/profiling the users who consume them, now he will have made a fair bit of money in all these years so I'd say he already reaped the benefits of his own work, that doesn't mean he's not allowed to make more money off it, but as long as his work was based on another proprietary platform (Reddit) I'd say the ethics of it were in a gray area, kind of understandable, now he adapts his app to a new platform (Lemmy) that doesn't monetize its users and is libre software (can't go further than AGPL) and he keeps his app proprietary and with the very same downsides as before, not only that, it costs more, even though there are no API costs and the transition to it couldn't have cost him some ungodly amount of effort to implement, but that's beside the point, everyone names their price, I may not agree with it, but it's not a bad thing per se. What I'm not ok with is how he's half profiting off some other people's work, i.e. the server developers, I think it's morally wrong, but I would have never thought that if, for instance, he made it open source with no ads trackers and sold it for any amount (20€? 30€? Idk) on the Play Store. You probably wouldn't agree with me simply because ethics are subjective and free software is a matter of ethics, not money, I care about it because, to a certain extent (I'm not a maximalist) I care for my own freedom in computing and black boxes just hurt a free ecosystem.