Developers don't convince anyone of anything! They just build stuff according to standards (which are inevitably set not by standards orgs, but by the most popular browsers), and then they expect all browsers to follow those standards and "just work".
When a browser like Safari fails to adhere to those standards, sites will break ... but you can't expect developers (of most sites; I'm not talking about the top 100 or anything) to test in every possible browser ... and then change their code to accommodate them. Certainly not in ones with single-digit percentages of market share, that require their own OS to test (like Safari).
If Apple wanted more web devs to support Safari they should port it to Linux and Windows. The web is supposed to be an open standard, you shouldn't need a devices and software from a specific manufacturer to develop for it (I say that posting from a Mac).
When a browser like Safari fails to adhere to those standards, sites will break ... but you can't expect developers (of most sites; I'm not talking about the top 100 or anything) to test in every possible browser ... and then change their code to accommodate them. Certainly not in ones with single-digit percentages of market share, that require their own OS to test (like Safari).