> paying for proprietary software does not mean any right for changes as such
Sure it can. There are enough cases where the rights of proprietary software are transferred to the client, or the contract includes the right to make changes or get changes made.
That's not at the scale of MS, of course, but that's exactly the kind of qualification missing in OP's statement.
What do you mean by that? With OSS, I can demand any change that I want and have any software development shop that I choose implement that change at a competitive price.
Quite on the contrary, paying for proprietary software does not mean any right for changes as such - even when paying for really expensive enterprise support contracts; I recall a case where not only the vendor would not make such a change, but we were prohibited by the contract to make the change ourselves (by patching the compiled code, which we were technically capable to do for that particular change we wanted).