The 10% that witnesses get is not for development, it's for securing the chain. They need to be on call 24/7 in case some action needs to be taken. I think that a top witness gets about 30k USD per year which is peanuts compared to smiliar positions in the tech industry and of course they need to be able to read the code and test it.

Without them there is no blockchain, there are no posts and there are no rewards for everyone else. That is why they get paid. Ideally they should have the knowledge to propose changes to the code. It's not a prerequisite but it's definitely a plus (for example aggroed is not a developer). Our job as stakeholders is to vote for the witnesses that will run the code that represents our interests.

EDIT: I just realized that you might be refering to the SPS and not the witness pay. The SPS is not for developers it's for any proposal that could bring value to the network. Marketing, producing tutorials are examples of non-developer related projects that could be funded with the SPS.