This won't happen. The Wii U was dead by 2014, and with the production of the console now officially over, it would be insanity for them to release it on Wii U. Why do it when that will actively harm the Switch's appeal and sales? As a business they'll make more money from people jumping to Switch for the game than a few thousand buying it on Wii U. On top of that Nintendo has already stated that Breath of the Wild is the last Wii U title from them.
The thing is you need to realise games won't be released on a dead/last gen console (last gen for Ninty) just because a small handful are unable to afford the new console in its launch year. I don't want to be rude or condescending, but this comes across as a childlike thought, where it doesn't consider that reality will never work out that way. I for one am glad that Splatoon 2 is going to Switch. It is coming in the launch year, Switch is enticing and there's more time for the player base to grow bigger and better than the Wii U's and regular updates are likely to go on longer than the original, so more changes to the meta via balance changes, additional maps and weapons amongst other tweaks and changes. I myself will struggle to get a Switch and Splatoon 2, but I will try my best to get the money together for it when it launches in the Summer, so there's time to save up for it. I can see Nintendo making a Switch + Splatoon 2 pack-in bundle, with a likelihood discount of the console and retailer bundles.
