Never too late for them to make a go for it, even today.

But, to avoid alienating their huge existing developer community, they would first do well to get behind Mono [0] and maybe even, make it official (edit: Mono is "Sponsored by Microsoft", so already quite official indeed). In fact, as of 2014, there have been very promising signs in that direction. [1]

[0] https://www.mono-project.com/

[1] https://opensource.com/business/14/11/microsoft-dot-net-empo...