So at the moment I see no reason why Go written BGP would be better than standard Quagga/Zebra. There aren't really concurrency or resource issues with large scale Quagga in my experience.
Really, you're playing on a semantic ambiguity in the word "router". A BGP implementation doesn't forward packets; it maintains a database of forwarding paths that the packet forwarding layer consults. In a large Cisco router, the SOC that runs BGP and maintains the RIB isn't the same electronic component that forwards packets.
Standard network software such as Postfix and OpenSSH took ten years to replace their predecessors, and their eventual replacement will be just as gradual. It's not happening right now, so I think it's a bit of a stretch to call it a trend.
And then the fact that safe, reliable code is only first step toward secure code where an intelligence, malicious person is targeting it. Totally different ballpark that neither Sendmail nor Cisco handled so well. Small shops like Sentinel and Secure64 did way better with a tiny fraction of the money. So, it has to be intentional for the extra profit at customers' expense.
Off-hand, you could use GoBGP to do cheap loadbalancing-ish things without external dependencies.