I agree with most of what you say below regarding linux sysadmin, BSD etc. I'm quite happy and actually would prefer building a linux solution on our own hardware. However, politically I think this is going to be difficult. I just feel that they will be more comfortable with embedded network boxes as a pose to a linux solution. I guess what I'm saying is this is partially a political thing.
Most purpose-built routing "appliances" use ternary content addressable memory (TCAM) in order to accomplish deterministic, hardware-based, longest-prefix lookups in large routing tables, such as a full Internet BGP feed. TCAM is used to replace software-based table lookup algorithms which have been empirically shown to lack scalability as the number of routes in the routing table increases, and as the line rate in/out of the routers increases. Current TCAM hardware-based routing engines scale to 10 Gbps, and beyond. Much research has been done in this area in the last 15 years.
I don't think general purpose BSD/Linux systems meet the scalability requirement for deterministic network design. Nothing political about that.