The Dangers of Building on Someone Else's Platform
trevoro.ca
trevoro.ca
It's got nothing to do with the vendors being hostile nor is it even remotely curious or in any way novel. It is the way industry works. An enterprising someone finds a niche and either grows the niche and the niche is either paved over or absorbed. Rinse repeat etc.
Look at the histories of Google, Amazon, Apple, Facebook, eBay, Adobe, IBM, HP, etc. All of them had complete, standalone products right out of the gate. More to the point, they took the revenue from that product and then moved to ruthlessly and consistently 'fill gaps', displacing other businesses along the way.
If your business depends on somebody else's "platform", then you're quite likely living on borrowed time. It might be okay as a way to bootstrap, but I'd recommend getting as independent as you can, as soon as possible.
Trick is figuring out how much time you'll need to move out from under the shadow of the service you are hooking on to, and that I'm not sure if can be easily figured in many situations.
I think starting by improving an existing moustrap gives you enough of a headstart over startups building entirely new mousetraps, that once you have market share and profitability, you can branch out into other areas.