3700$ is more than enough for the infrastructure, but as you said correctly, it takes much more than "infrastructure" to be a good great witness. The invested time is quite high, especially since witnesses are supposed to be involved in everything related to Steem, as well as providing/developing tools.

On top of that, it's also somewhat risky to "become" a witness. At the beginning (2016), it might have been easy, but over time, the requirements have only risen and it has become more and more difficult to reach the top (especially TOP 20). The amount of shuffling among the TOP 20 is very low (which is good for stability), but bad for newcomers. (Many backup witnesses can sing a song about that).

So regarding the post's question: If the revenue would be as high as before the 80% witness-cut, then sure, rewards could be split. There are pros and cons to that subject. But right now, economically speaking, it's not viable. And there is, of course, the question, how low you can go before talented witnesses decide it's not worth their time.