False equivalence - a manual gearstick complicates the design of the car, making each unit cost more, while allowing sideloading is a one-time development cost with marginal maintenance costs. Also my car manufacturer doesn't get a 30% cut every time I fuel up because I use an automatic transmission. If it were like that I can assure you there would be a much bigger uproar than in this sideloading debate.
It's hard to imagine how a working developer can believe that maintaining a platform where anyone can do anything has only upfront fixed costs. Apple spent about 4 years trying to change the contract of the pasteboard API to resolve unwanted data access concerns.
The Play Store has similar fees for IAP and sales as Apple. When people complain about Apple charging developers, it’s not about the $100 annual cost of the developer program.
They aren't complaining about the 30% either; they simply want a way to compete with what the App Store charges for. Android provides that, iOS does not.
There are people who complain about the 30% as well. As soon as the Microsoft Store was fully dead, Microsoft started saying that 30% for a store was too high.