If clients supporting multiple protocols become popular they could default to a sensible open protocol. Which, as the default in the popular app(s), would become the most widely used protocol. People can switch at their leisure because the app supports both, but over time the users transition because it's the default.
Since the old and proprietary protocols are only sustained by the network effect, they lose users, and eventually support for them gets dropped.
It's a method of transitioning from a collection of proprietary systems with their own network effects to an open one, by temporarily supporting both. For obvious reasons the operators of the proprietary systems don't want to be subject to that competition, which is they same reason they shouldn't be allowed to shut it out.
Since the old and proprietary protocols are only sustained by the network effect, they lose users, and eventually support for them gets dropped.
It's a method of transitioning from a collection of proprietary systems with their own network effects to an open one, by temporarily supporting both. For obvious reasons the operators of the proprietary systems don't want to be subject to that competition, which is they same reason they shouldn't be allowed to shut it out.