I think a lot of people were expecting sales of Super Mario Party to crash once Mario Party Superstars came out. So I’m not sure it’s going to reach 20 million now that there’s another game out in the series.

Super Mario Party has had three years worth of evergreen sales already and could continue to sell for the rest of Switch's life, Superstars will simply have less time to reach its final sales.

@ShadowLink93 Where did you get the Top 30 graph? I'd love to see a Top 50 graph.

I find NSMBUD sales painful too, in that it hurts to see such an old and mediocre game selling as well as it is when there are so many far superior games more deserving of such sales, but at least it's not selling like Mario Odyssey or MK8D.

I don't get it. Why do people think NSMBUD is mediocre or bad? It's a highly polished great looking game with really good level and overworld design. It does lack genuinly grear new ideas, the bosses are the same old stuff and the soundtrack is uninspired, but apart from that it's high quality.
What's the issue? I constantly heard people dumping on it, then I got it and was genuinly surprised by it's quality. I just don't get the criticism.

Just speaking personally, I found it overwhelmingly lazy, uninspired, and bland.

The graphics were poor by Wii U standards much less Switch, art design and music were soulless and tacky, level design was forgettable, the controls were not as tight as the old 2D Mario games.

I agree on the music part, but apart from that... well, nah.

My take on NSMBU is that it should have been a Wii game. The only change would have been 480p and it would have been a great swan song and sold a ton on that console. I think a lot of the criticism comes from people just being tired of the "New" games by then. Some were better than others, but none of them had the charm of the originals (minus Lost Levels). NSMBU is a good game and I did really like the overworld, but the squirrel power-up was pretty lame and I do have to agree with @curl about the visuals for a WiiU game; playing Tropical Freeze after Mario was a night and day difference.

NSMBU is a good enough game on its own merits. It's just that people are sick to death of the NSMB series, myself included.. The aesthetics, the gameplay, the music, the whole enchilada.
It was great back in 2006 when there hadn't been a new 2D Mario game in 15 years. But then Nintendo milked it to death. To the point where we had 2 of them releasing within 4 months of each other.
Just... No... No! Enough! No mas! We're sick of it!

I think it's like Fifa, after too much games released with the same concept some people just get bored. Something I don't like about it is the presentation, same music, art style and background

However I need to address at this point is very likely that nothing Nintendo releases will satisfy me in terms of 2D Mario, thats because I was never a deep fan even of their DS/3DS games when I already felt the New Super series was nothing but a shadow of their 80's and 90's counterparts

For me 2D Mario is already expired by definition, it's an 35 years old franchise that peaked in the early 90's and since then provided very few gameplay improvements. For a 2D Mario being worth it will need to revamp the gameplay entirely and losing it's DNA to become something else which Nintendo will never do. What do I mean by losing its DNA? Think about something like Celeste, it's a 2D side scroller but plays completely different from Mario that is an action game with accessible mechanics while Celeste is more focused on slow movement, complicated controls and high precision 

But of course, I'm a minority as the U Deluxe sold over 10 million copies after all