I'm hesitant to pick up another network protocol after OpenFlow. Lots of vendors claimed to support it, but only a handful took an honest shot at a decent implementation.
Just to be clear, p4 isn't actually a network protocol. It's a programming language, a DSL for programming network protocols. Small but significant difference between the two. But you're correct that it will require native support among router/switch vendors to be worth using.
Actually, p4 is aimed a solving a lot of the problems with OpenFlow that led to the situation you describe: incomplete support by vendors of all the different versions of OpenFlow. That's because each individual network protocol supported had to be written into the OpenFlow spec, thus the proliferation of versions. Whereas with p4, the vendors basically just provide a hardware runtime for the p4 language, and you install whichever protocols you wish to support (or write it yourself, if it's custom or new). It also has the potential to work very well with a scaled-down version of OpenFlow.
I'm not an SDN expert and am probably not doing justice to p4 with my description above, but it is important to realize this isn't just another network protocol. It would be like describing the Python or JS as just another program (the interpreter), and not as a program capable of running other programs.
