But that's incidental to what was my real point, which is that if you do want a separate network, it has to be separate. Tie your two "separate" networks together with integration and you're returning single-points-of-failure back into the mix. Granted, practicality may dictate a couple of those... integrated authentication comes to mind... but you don't want the two separate networks to be all slickly and smoothly integrated for the most part.
Regardless, statefulness is bad of course, and less of it you have, the better you are off - we've seen this time and again: share nothing arch[1], Amdahl's law[2], locks-vs-RCU[3], memorizing tcp flows-vs-IP header hash[4,5], etc... Being able to make decision on current packet/memory access here and now, independently, without having to keep track of other flows/connections/sessions/threads beats alternative approaches every time [6].
[1] https://en.wikipedia.org/wiki/Shared_nothing_architecture
[2] https://en.wikipedia.org/wiki/Amdahl's_law
[3] http://www.rdrop.com/users/paulmck/RCU/hart_ipdps06.pdf
[4] https://tools.ietf.org/html/rfc2992
[5] http://www.juniper.net/techpubs/en_US/junos13.2/topics/conce...
[6] Intel's heroic effort to maintain cache coherency at all cost is a notable exception, although opinions vary wildly on whether this is actually the best thing to do, and how long they will be able to sustain it.
The answers is the same as in software industry: Configuration management.
Plus it makes then harder to develop corporate cultures of deploying well known patterns that makes routers best used.
Plus it increases the surface of vulnerability for government backdoors.
In real life, there is a trend for going BSD box for cost/formation/stability/security reasons in the ISP world ... especially in non US countries that are getting kind of thinking the USA is becoming the new USSR in terms of systematic spying on every one.
These Europeans really don't get it, isn't it? They never were in the middle of cold war spying insanity where both power would puppet so called terrorists groups to make sure citizen would vote the right way under the fear.
What a bunch of stupid persons not believing that business companies are incorruptible and think they have incentive to betray them in exchange for gvt subsidies/regulations.
Or with a generous budget, you can do what BT have done with their 21CN project: For each macro-block (access, aggregation, core), have two redundant networks each one built by a different set of vendors.