There are many options for you as an app developer. You can enjoy the scale and tooling provided by Apple, at one cost. Or you can write apps for any other computer at some other cost.
Or you may choose not to be an app developer at all and do something else useful.
Apple doesn't have a monopoly on phones. They've just dictated how their phone is going to work.
The options aren't Apple iOS ecosystem, or no phone dev at all. There are numerous other smartphone companies out there. Apple isn't even close to the largest share of operating systems used in the market—by far (https://www.statista.com/statistics/266136/global-market-sha...)
There are many options for you as an app developer. You can enjoy the scale and tooling provided by Apple, at one cost. Or you can write apps for any other computer at some other cost.
Or you may choose not to be an app developer at all and do something else useful.