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

The only reason they are doing Zelda on Wii U, (if they even still do) is because they promised it for that console so many times and don't want to incur the wrath of their core fans. The next big Mario game was never promised for Wii U, so it makes no sense to release it there.

Not if the install base isn't there. If the option is to try to sell it to 4 mil on NX or 17 mil on NX + Wii U, it would kind of only make sense. 

Wii U would not offer a substantially bigger player base though, because by the time NX is out, Wii U will be effectively forgotten, people will have either sold their Wii U consoles or stowed them away in a box in the attic. They'd gain very few sales by adding a Wii U version, and they would be limiting the ability of such games to move NX hardware, which is much more important to Nintendo's future.

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/

Now you're just assuming. People didn't stow their PS3 and Xbox 360's away in the attic, and there were still games coming to those platforms for a while. Not everyone is going to be able to afford an NX at launch and sometimes it takes a year or two for the price to drop before people can afford it. Sure we are fortunate enough to be old enough to have jobs and be able to buy consoles fairly easily, but we aren't everyone.

PS3 and 360 have 6 times the install base of the Wii U and kept players supplied with software even after their replacement. Wii U will be abandoned more quickly because hardly anyone owns one anyway and it's offering basically nothing between now and Zelda to keep players engaged.

Wii U will more than likely still have a bigger install base than the NX for a while (say roughly 2 years).

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.

Yet again, only for those of us who can afford consoles. Not everyone is that fortunate. Besides, didn't you (as well as I) say we aren't going to be early adopters of NX unless something amazing is shown? And it was more than just us saying it.