Because it didn't fail yet, it's still a work in progress. And there were a LOT of people who complained about the old model. I suggest you take a look at the perchance commutinty on reddit, there are a lot more people than here, but even on lemmy you can see it when you take a closer look, for example: https://lemmy.world/post/36459439 atm, 3 up votes, 7 down votes. People do like the new model when it doesn't have hiccups due to finetuning the settings.

About letting people switch, I already mentioned it in a much older post: dev would have to set up the infrastructure first, which would probably take a lot of time. Maybe dev rather use this extra time to work on the new model. After all, it's already almost 2 months and you can see how people get impatient bc they don't understand how much workload this is for a single person.

As for the ads, dev deliberately chose to make them as minimal as possible to make it as user-friendly as possible. If ads get more invasive, people start to use adblock, too. IMO, perchance is a gift by dev to us, and maybe dev even takes pride in giving it completely free. That's the only explanation I can imagine why dev doesn't take donations, bc many people mentioned they'd love to. But even with such a big heart, dev faces limits in resources as time and money (and probably energy), that's why we don't get the old model.