I wouldn't say it affects the chances of Nintendo releasing a Wii U Sports game. The Wii had several third party sports games like that, but Nintendo still published two Wii Sports titles.
I think there's a decent chance of a Wii Sports U game being released as it's a game that would appeal to a lot of people, especially casual gamers. If Nintendo released it as a downloadable title for £10 or less, then I think a lot of people would buy it.
This is the right idea, not only is Wii Sports as a retail game baffle me (which it was in Japan). Having the "casual" gamers set up online accounts to download a game like that will expose them to the other downloadable games and increases their numbers.