> would collect some fixed amount of money from enterprises in exchange for support on all OSS projects said enterprise uses (limited by hours per month?).
Yes, if you developed Keras and enterprise is Google. But what if enterprise wants to use some small plugin for Wordpress which is opensource, and needs some new functionality or a bug fixed? It seems unlikely that each enterprise will offer a job to each of the OSS maintainers, even if the said persons are even willing to offer such support.
That's called getting a job with said enterprise.