How out of touch are you? The special editions for this thing are sold out. Mania is hyped as fuck, the reception to it is so overwhelmingly positive. A lot of my friends who don't even play sonic games are interested in it.
NSMB is not a fair comparison as that shit was just the same rehashed game we've gotten several times over the last few years. Sonic mania does its own thing and feels like a new game in the classic series, it's like the Sega Saturn game fans should have gotten.
Not to mention it has really revered people working on this who have done excellent stuff in the past. It's looking good because it's obviously a passion project for them, they completely understand why classic sonic was so enjoyable.
To think mania won't sell is laughable, it's very obviously going to be incredibly successful.
Maybe try researching what you're talking about because there is no way you could have come to the conclusion "mania won't do well" if you had been paying attention to it at all.