Why should professional Studios quit Unity?
Really? They are the ones that are affected the most!
Maybe ask Massive Monster why they plan on ditching Unity and threaten to delete their games from stores. They have a discord, I bet that one is buzzing right now.
My guess is, that is has to do with principle and not wanting to have unreliable business partners and financial risky calculations. The business is hard. And Unity is not applying the fee on financial success and sales, but on installs. That is not the same.
If you lack the imagination, think about scenarious involving pay what you want, free with adds/premium, bundles, discounts...
In other words, if no one were affected by this pricing model change, why would they try to implement it in such a way in the first place? This is costing developers more money while not giving them more benefits. This alone is reason to be angry.
In yet other words, why should a gaming company put up with a engine maker that changes pricing so, that your licensed engine now ramps up from costing you 10k $ a year to double or triple or more? The pricing for Unity pro currently is ~2k per seat a year. So having 5 developers developing in that one would cost 10k for the engine alone. Developers want money too, so your company must make enough money to employ 5 devs. Plus assets and whatnot. So you can assume that your games have to have a sales volume putting you over the thresholds. 1 million times 2 cent gives you an additional cost of 20k. And even with only 100000 that makes 15k.
Also Unity has demonstrated that they might radically change pricing. So they could change it radically again. Lowering thresholds, increasing fees, removing exceptions, counting differently, and so on. And on a short notice. This is hard to calculate. And if you try to have a company providing your livelyhood, you want stability as much as you can. Players are fickle enough.