[Diggsey]

You're suggesting that being liberal in what you accept is necessary for forward evolution of the protocol, but I think you're presenting a false dichotomy.

In practice there are many ways to allow a protocol to evolve, and being liberal in what you accept is just about the worst way to achieve that. The most obvious alternative is to version the protocol, and have each node support multiple versions.

Old nodes will simply not receive messages for a version of the protocol they do not speak. The subset of nodes supporting a new version can translate messages into older versions of the protocol where it makes sense, and they can do this because they speak the new protocol, so can make an intelligent decision. This allows the network to function as a single entity even when only a subset is able to communicate on the newer protocol.

With strict versioning and compliance to specification, reference validators can be built and fitted as barriers between subnetworks so that problems in one are less likely to spread to others. It becomes trivial for anyone to quickly detect problems in the network.