This isn't a black and white issue. Some degree of compromise is needed as neither side is 100% right. What seems reasonable is a degree of resiliency be built in - so if the protocol worked on a subset of features, you don't brick it as you expand to new ones, you merely degrade.

It's not a perfect analogy, but the rules on radio interference with devices spring to mind: you need to cope with some interference and you need to not emit it. This results in a more stable ecosystem, even though you could sensibly argue that if one side is stuck to absolutely, the need for the other wouldn't exist.