Shit, I can build a lot of redundancy with four used/grey market $17,000/piece juniper mx960, $20,000 for rectifiers/battery... It's not THAT costly relative to $4m/mo even if your router requirements are as beastly as an asr9910 with 3rd gen line cards.
In networking, redundancy means two identical boxes with identical bugs.
There are very few companies that are willing to purchase network gear from two different vendors and run two completely different network stacks.
Management would be a huge pain in the ass... but it would be doable.
We are a large regional ASN in a tech heavy market and run 3 different vendors of DWDM platform and both Cisco and juniper in the IP core. Diversity is good.
Plus a third box for testing firmware updates before you touch the live ones.