Thinking that if there is a "pro" that it must be followed by a "true successor", as if these devices have to slot nearly into either category or follow the PS/Xbox line to the letter, is imo misguided. The Wii was a new generation and the new 3DS wasn't, the latter was a bigger jump in power, correct me if I'm wrong.
Game engines are scalable, Nintendo is still selling their evergreen titles at full price and adding DLC, and they intend for households to have multiple Switches. We are aware of only hardware config from the Nvidia leak using NVN2 and that's Drake, a "generational" leap in specs but will probably be sold as another Switch. It'd be equivalent to a new gen with an extended cross-gen period and enhanced backwards compatibility. It is Nintendo who has to do the heavy lifting to market it properly and eventually phase out the OG Switch. So if someone says "Switch pro doesn't exist, it'll be Switch 2" I don't think that changes anything about the actual hardware we should be expecting.
Im also not convinced that any Switch successor will have a new main gimmick just because Nintendo had the whole Wii to Wii U or DS to 3DS thing. I think hybrids are here to stay for a good long time and they can change the OS, joycon, and display or add features like wireless streaming and assymetrical multiplayer without ever abandoning the Switch concept. Nintendo has very explicitly learned from the mistakes of the Wii U. I expect a VR/AR device in the future as a third pillar to Switch and mobile, not a replacement.