Billy does a lot of the podcast mixing/engineering/production work and with that being the future, he's probably safe.
I think Roy does the ads (and I'm just going off him getting blamed, whether right or wrong, whenever Gotz fukks them up). He's also the only one other than Mike that can handle their production boards.
I don't know what the value of those are, since I'm not in media/radio, but I imagine he's not that easy to just let go.
In terms of what they do on the show I think the only guy they "can't" replace is Stu. But it doesn't really matter, all of the crew is too attached to not get Dan severely pissed off, throwing a tantrum on air and make ESPN look like shyt on live radio before he gets suspended/fired
Roy = He started as a fan and worked his way up to be a n°2 to Mike. If he gets let go Dan is gonna act like they took away his child or something
Guillermo = I don't think Dan loves him like he did Chris Cote (who he knows since he was a little kid) but I think he sees more value in him not just as a producer but as a personality. 1 out 10 things Billy does on air actually work, but he has a Cote-like ability to make his failures fun to laugh at.
Mike = He's Dan's errand boy and for or better or worse is the one that deals with all the personalities on the show. If he gets fired Dan probably doesn't show up to work the next day
Stu = Dan would laugh his ass off and then go and basically burn every bridge with ESPN because they killed the show