We have to be clear, what is the point of the Switch. Switch is not simply a hybrid, that can be played mobile or stationary. The main point is the unification of the two game development platforms Nintendo has. The hardware is only to accomodate for the different playstyle these prior separated platforms had. Additionally Nintendo tried to get also close to smartphones to get their games too. Looking at the lineup this massively succeeded, while the mega-big AAA-games are missing, the Switch has a lot of lower profile home console game, classic handheld games, PC-games and also mobile games. The unified platform gives the devs the benefit of the bigger userbase and out of the box different playstyles and Nintendo the benefit of a diverse library of games.
Actually Sony is in the worst situation to cough up a competing product. It cannot be the PS5, as mobility would kill the power and people would be severely disappointed, even alienated from the Playstation platform. So it has to be an additional product, which instantly means they would have two different game markets, which was the whole point the Switch was made to avoid. So missing out on the unified platform, game-dev would have to produce special version, as they did for Vita. These are unattractive for gamers and because of new development for devs too.
Microsoft seems to be not too interested in this market. At least with the Nextbox I see them make a competing product to PS5 and this would mean the same as for PS would apply.
Some newcomer on the other hand would have a good chance in this market. Going after mobile games and indie PC-games to create an initial library such a platform could look good. With advanced specs in comparison to the Switch it could present itself as the better option. Even better if a classic company like Sega or Atari would go for it. They could add to the library their old retro games as additional advantage.