It seems rather silly to say "I think Square-Enix hitching their wagon to Sony has caught up to them.". What was their alternative? A WiiU version would have to either look and run like balls, or required them to fundamentally change the game across all platforms. The former would have been pointless, and i'd argue the latter's overall effect would have been almost entirely negative for the game. The AAA market in the west (FF's most important region) is far too competitive for them to allow major technical compromises for the sake of an overall relatively unsuccessful console.
SE 'hitch their wagon' to Sony because at least as far as Final Fantasy is concerned, they're their most profitable global partner. The FF brand continues to be best associated with PlayStation (thanks to their partnership with them during the 5th and 6th generation, which worked out spectacularly for both of them), the PS4 is the current market leader (by a significant margin), and all currently available handhelds lack the power necessary to delivery the AAA experience people expect from Final Fantasy. SE's only currently existing options outside of PlayStation is Xbox (which they've already established as somewhere all core FF entries will be available), and PC (a space they're quickly growing into).
I agree a Switch FF15 release is worth considering, but the way you're presenting your argument feels rather hyperbolic. Past decisions should not be judged by the assumed potential of something that couldn't even be factored into those decisions.
