I love Mario Maker 2. It's an enjoyable tool to use, and one that's possible to make some really cool levels in it relatively easily, or with a bit more work make some really oddball contraptions that can be used in creative ways that don't really feel like Mario. It's also possible to play some awesome levels that Nintendo would never make, be it because of their difficulty, they become significantly less interesting if Nintendo put them into a 'real' Mario game, the level is telling an interactive story rather than actually trying to be a 'real' level, or the level fundamentally doesn't feel like Mario for some other, more subtle, reason. I have probably gotten more enjoyment - both in terms of hours of entertainment and level of enjoyment - from Mario Maker 2 than I have the four New Super Mario Bros. games combined.
The general game-playing public, for all the vocal people online who seem to prefer Mario Maker to New Super Mario Bros. and perhaps traditional Mario generally, seems to prefer to spend their money on traditional Mario. As of the end of December 2019, Super Mario Maker 2 had sold 5.04m copies and New Super Mario Bros. U DX had sold 5.85m copies, and while it has had about six more months than Mario Maker to sell those copies, it's also outpacing Mario Maker 2's sales - 1.26m copies sold in Nintendo's 3rd quarter compared to Mario Maker 2's 1.11m. Meanwhile, on Wii U, the original Mario Maker sold 4.01m copies, and the basic version of New Super Mario Bros. U sold 5.79m copies.
For all I prefer Mario Maker 2 overall, I don't think that Mario Maker replaces traditional 2d Mario. I say this not because I dislike the infinite possibilities provided by Mario Maker, but instead that I believe there is something fundamental missing from Mario Maker that traditional Mario provides, and not simply that Sturgeon's Law is in full effect for Mario Maker, though the good stuff is often golden and via the use of following level creators you like the levels from, and the ranked lists, it's fairly simple to curate your play experience to mostly avoid the bad stuff.
Instead, what I find myself missing the most is the lack of continuity. There are two ways of playing user crafted levels in Mario Maker. You can either pick them off of a big list of levels, sorted into tabs like New, Popular, and creators you follow, or you can play Endless, an infinite random grab bag of levels categorized into four difficulty modes, where you play until you run out of lives, and can pause the experience at any point.
In the first case, picking an individual level, lives and coins have zero impact, you have infinite lives to beat this one level, and no matter how much you collect from the level you have nothing but your memories when you come out. In the second case, you start with a finite number of lives, and while collecting extra lives and coins matters nothing else carries over from one level to the next. Rinse and repeat with the next level, the difficulty of which will not have built on the last, and whatever power-ups you left the prior level with will have been lost. Either picked by yourself from a list or randomly drawn for you by the game.
Contrast with traditional Super Mario - The exact mechanisms of how levels connect to future and prior levels have changed, all of them connect in a way that feels connected, and power-ups have always carried forward to future levels. In the case of Super Mario Bros. this is as simple as it comes, a linear series of levels with occasional warp zones, if you know how to find them, allowing you to skip a few. Lives and coins are universal, difficulty gradually builds over time, and if you finish a level as Fire Mario, you start the next as Fire Mario. Super Mario Bros. 3 expands on this by adding a world map to proceedings, allowing you to use stored items on the world map, skip levels or take alternate routes by navigating the map differently, and so forth, with Super Mario World taking this even further, by giving some levels secret exits that unveil alternate paths and shortcuts throughout the world - Some levels are hidden on the world map until you figure out how to find a more obscure exit, and completing some optional levels makes other levels easier. New Super Mario Bros. never quite gets that cool with how continuity between levels is handled, but the continuity is still present.
Showcasing Mario Maker 2's continuity in the best possible light - The Story Mode - The game presents you with a level, where collecting coins helps unlock levels faster. Lives are local to the level, and like with regular levels, any power-ups obtained will not carry over to future levels. Even more of an issue, you're ultimately still selecting levels off of a menu, with no build-up of difficulty since, while levels of multiple difficulties are present, the different difficulty ranks don't feel ordered, and you can unlock different sets of levels in different orders. Less linear level structures can work but the way you get levels here just doesn't quite work for me. As a value-added for Mario Maker, it's great, but it doesn't replace the feeling of playing a coherent game that traditional Mario provides.
Could Mario Maker replicate this missing coherence of structure in future? Absolutely. It only needs one additional tool, though not one I expect to be added in an update or paid DLC - The ability to put levels into the context of a game, where power-ups are kept from level to level, even if only as interconnected to each other as in Super Mario Bros., but ideally with a world map similar to in Super Mario Bros. 3 or Super Mario World. Possibly with the ability to tag levels so they do and don't appear outside of the context of those games. But without that? As wonderful as a tool as Mario Maker is, I just can't see it being able to capture the cohesion of a traditional 2d Mario.