I suspect it underperformed as a game
It was literally the best selling 3D Mario game, 4th best selling mainline as a whole. Hell, probably the last game where I genuinely saw excitement from kids regarding Mario that wasn’t the movie.
It's not on the level of Super Mario 64 or even Super Mario Sunshine where people still talk about it for years afterward.
New Donk City is now a staple of the series and Pauline is now a mainstay. Even the Mario Movie had call backs to New Donk and Tosterina. The only reason we didn’t see a sequel is that the team moved into Donkey Kong afterwards. Only real complaint I can give is how little Odyssey impacted the spin-offs, but that has been an issue since NSMB overtook the franchise and booted all unique content.
All the companies blamed COVID for delays, even though video game development is one of the few careers where WFH will work without jumping through too many hoops, and Nintendo doesn't seem like one of those companies that can be accused of ending WFH for seemingly arbitrary reasons.
If I remember correctly, a lot of Japanese studios struggled to move into WFH due to company culture and usual Japan weirdness. It somewhat marked a death period when developing titles till things got back to normal.
Clearly you underestimate how difficult it is to turn the color blue into the color green. That's a lot of shrines to change the color of, and all those korok seeds to collect... truly they needed double the time it took to develop Twilight Princess to do that. In all seriousness, taking 6 years when they got to reuse the entire world, the characters, the items, and even the plot delivery method is crazy inefficient, especially considering how empty the sky and depths turned out to be. They fooled me with BOTW, but I didn't touch TOTK.
Going to be nice to TOTK, mechanically, the game seems to have some rather complex systems in place regarding the build mechanic and all the various eccentricities of it. Not a developer at Nintendo, but won’t be surprised if they rebuilt much of the game to work with Ultra hand, fusion, recall and the other various new mechanics which seem to interact with everything in the world.
The physics in the game are insane and had to be a nightmare to program. The build mechanics had multiple points of connection in each object. Recall had to have entire systems set up to pin down the physics and movement tracking of each item even fusion needed to have each object set up to strap all other objects onto it and use their various effects. This is all without going into Zonai devices like the steering wheel, which turns every object into something controllable and with incredibly smooth directional controls that just work. All this within an open world running on a god damn potato.
The game is way more than a DLC, only issue is all the new content is stored in the hood and not present on the forefront of the package. The long wait time makes way more sense when you look to how multifaceted every system in the game is and imagine it running on the Switch.
The biggest failing of TOTK is just that they got too caught up creating the mechanics and not enough time was spent building up a world to house them. Ultra Hand, Recall and Fusion all feel like afterthoughts for the average gamer to make use of while playing. They are incredibly impressive, but more so as a baseline for future titles to make use of rather than the game that actually came out.
