[Someone]

> If, as a developer, I knew I only had a couple of years at best before Apple replicates my app, would there be any incentive for me to create the app and pay Apple the $99 yearly fee?

That “couple of years” could be enough incentive.

> As Apple isn't obligated to provide anything good in terms of software, just a free alternative that's baked into the OS, would that even matter in the end?

Nobody is obligated to provide anything good, but as long as there’s competition, the market will force suppliers to provide good enough stuff for the price they ask.