I can’t argue with the general point you made in your post, but I’m not quite as pessimistic as @liquidjoe or you, and would not say that the absence of Netgate in the OSS and/or FreeBSD world would “choke” projects like OPNsense to death, nor would I say, to use your nicer term, it would be inevitable that they will “fade away”. 
As for contributions, my thought process was that increased demand would bring more funding to these projects, and with the additional funding they could hire more full-time developers who would then also do more contributions to FreeBSD. But maybe that’s just wishful thinking on my part.
But hey, as of today, the community edition of pfSense is still around, although Netgate’s latest moves and statements don’t necessarily boost my confidence that they’ll keep it around forever. 