With this limitation in mind, an upstream component could still terminate
TLS, but couldn't Gorouter also?

Just as a small followup, consider that companies might have internal regulations mandating how and where SSL termination needs to happen (Rakuten is among them, for example...). As described in my previous mail, there are workarounds (e.g. using a separate TLS session between LB/RP and gorouter) but this may add further deployment complexity (and overhead).