Boycott Apple... and then what? What's the viable alternative? Android? No, thank you. At least you have some leverage over Apple by the virtue of being the customer. The competitor's business model makes it much harder to influence, which is hostile to the customer and their privacy.
If you think Android is really open, think again. Without Google Play Services and Play Store you are basically hosed.
There's a lot of options. Easiest is just don't upgrade -- stick with your old phone as long as you can, and don't buy any more apps. If you need to replace your phone and don't want to use standard Android, you can use an Android fork that doesn't depend on Google. (I've been hearing good things about LineageOS, but haven't tried it personally.) Alternatively, you could try a less popular mobile OS like Tizen. Another alternative would be to use an older style non-smart phone. You could even ditch the phone entirely and go back to the way people lived before the last decade or two if you want.
Just don't give Apple any more money until they change.
Fair points. I don't think they are going to effective though for an average enough user to change Apple, and that just addresses the customer angle. As a developer you don't have much choice to boycott Apple.
You have to pick your battles in life. There are a huge number of companies I have chosen not to do business with and a long list that I'd boycott before Apple. I admire you if this is the cause you want to spend your life fighting for like I admire RMS but if that's the case one should really consider spending their energy ensuring a viable good alternative exists, and maybe lobby for regulations. Merely shouting 'boycott' in a niche community will not change Apple's behavior I don't think. This of course is not a discouragement to try, but a personal opinion on practically of achieving the desirable outcome. Especially so when the competition is much much more anticonsumer in a myriad of ways.
Android let's you install any app outside the store through sideloading. Imagine that, being able to install the software of your choice on the hardware you paid for!
FYI, it's been a while since you can get a signing developer certificate for free and sign and deploy any app you want for your own device. You can't easily distribute it though, as the complexity is equivalent to sideloading. It's a fair point that Android supports non-Google Play app stores and "Unknown Sources" app downloads. Google Play Services is a a very key tie-in though that's hard to avoid even if you're on LineageOS.
If you think Android is really open, think again. Without Google Play Services and Play Store you are basically hosed.