I think it's a bit unfair to say that Apple deliberately change interfaces in order to push their adapters.
They've got 2 different problems to address, firstly they want to update and be in control of the interfaces to their hardware, Firewire,Thunderbolt,USB-C but, at the same time their machines seem to last quite a long time, so at any time there's a pretty significant number of users stuck on older interfaces and with hardware that uses older interfaces too (e.g. I have an older Firewire external hd that I now dongle to Thunderbolt, have noooo idea how I'm going to get that to work when I get this new machine).
So in order to allow all that to work they need to support a multitude of interfaces because getting a new laptop shouldn't mean throwing everything else out.