I see Mario releasing on both platforms not being a problem at all for the system if Zelda isn't.

It is problem if they actually want to use max of NX hardware and that new 3D Mario not be held buy Wii U version of game. There is no single reason to develop any new game for Wii U that is not already announced because Wii U is already almost totally dead and now you can try to imagine in what statement Wii U will be at end of next year, most likely will be discounted. 

Also fact is that Nintendo never released game for previous console when new already arrived on market, so why would actually do that with their console with small user base ever when they hadn't done that before when that had much more sense!?

Only reason why they still releasing Zelda on Wii U also is because they announced Zelda for Wii U in January 2013 and basicly they working on Wii U version from that point, they also constantly talked about it, showed several times running on Wii U including two E3s.So after all that there would not be too much sense to cancel Wii U version. 

Also new 3D Mario almost certanly right aways started like NX only project, Miyamoto at end of 2014. said that new 3D Mario most likly will be on next console not Wii U.

https://mynintendonews.com/2014/12/19/miyamoto-next-mario-will-most-likely-be-on-nintendos-next-console/

This can essentially be said about every cross gen game. Lowering some settings on a Mario game however to get it to run on two different platforms should be WAY easier than say Assassin's Creed or Call Of Duty (the game is made with simple geometric shapes by nature).

We here not talking about any cross gen game, we here talking about Nintendo, I gave you clear facts why Nintendo will not make new Mario cross platform game.

Just because things weren't done in the past doesn't mean they can't be done going forward.

No it doesn't, but if have facts like Nintendo never done nothing similar in past, why would they do that now when they having worst selling console with lowest user base ever!? So we have facts like Nintendo never done something similar never before, Wii U is already dead not to mention next year, and Wii U user base is terrible, so you really didn't expect that next Mario will be cross platform.

And they basically said now that Zelda is only Wii U game for next year.

I will bet you NX first year won't have 13 million install base. 

They won't just be cutting out a few people here and there, they'll be cutting out millions (if Mario is released first year). 

No it doesn't, but if have facts like Nintendo never done nothing similar in past, why would they do that now when they having worst selling console with lowest user base ever!? So we have facts like Nintendo never done something similar never before, Wii U is already dead not to mention next year, and Wii U user base is terrible, so you really didn't expect that next Mario will be cross platform.

And they basically said now that Zelda is only Wii U game for next year.

Probably not, but NX will have life of at least 4 years, Wii U is already dead and have terrible instal base, next year Wii U will be probably discountend.

3D Mario is game that is selling good through whole life of console.

Like I wrote, It very unrealistic to expect Nintendo will release completely new and unannounced game for console thats is already dead after launch of new console, especially if Wii U is already dead and have terrible instal base. Basicly you dont have any reason to exept something like that, espacily when Nintendo this week said that Zelda is only Wii U game for next year.

Bigger in terms of the raw number of consoles in the wild, sure, but probably not a more active userbase. NX owners will be playing their new console and actively buying games for it to justify and try out their new purchase, Wii Us will be mostly forgotten and gathering dust.