Go back and watch every WWDC.

You'll see that they consistently say that iOS updates are always free. There is a reason they say that, it's not be accident or idle words.

It's because the idea of a free OS upgrade for non FOSS is non-standard. Android moving to paid, means they would be like the overwhelming majority of business models for OS updates (Windows, MacOS).

It also means that they were unable to successfully monetize/subsidize their OS over the long term - arguably the crowning achievement of Google - whereas Apple has been able to eat their own dogfood and make it more or less a singular product.