What you've just described is 2D Mario.
(lots of images)
They've hit a point where the technology allows their characters to look like their artwork. The world themes have remained the same across games, but Mario never really had more variety. SMB3 started the idea of themed worlds and then SMW pretty much threw that out the window in favor of SMB1 and 2's tropes. NSMB just brought it back. At least they put a lot of care into each and every world and level so that they are full of unique things and new ideas. Also, while they have pretty much standardized the art style, there are still significant differences between games and the backgrounds are never recycled. Level design has changed significantly from game to game, with NSMBWii and NSMBU sporting easily the best level design of 2D Mario. Gameplay has remained fairly stable across the board, with each new game adding new moves or mechanics. The most substantial change was SMB3 allowing the screen to scroll in more than one direction, as that opened up level design possibilities significantly. NSMB introduced new series staples like mini mushrooms that allow you to run up walls and across water and mega mushrooms that make you enormous for a short time. Also, being the first NSMB, it introduced many of new things that are taken for granted like rotating and moving level elements that would not be possible in previous games due to hardware limitations. NSMBWii introduced multiplayer and a jump augmenting mid-air spin move. NSMB2 put a heavy focus on collecting coins and getting high scores, which made it pretty different from any other Mario. NSMBU mostly improved upon what NSMBWii did in addition to adding boost blocks and challenge mode. And again, each installment added new elements that weren't feasible in previous installments. The amount and density of dynamic level elements increased greatly with each new game.
Except that in your example, NSMB Wii and NSMB2 are almost using the exact same textures for the grass. NSMB1 and NSMB Wii also share a very similar wavy-line pattern for dirt. It's not 100% exact, but it's definitely close enough. Gameplay concepts aside, they don't look very interesting visually. A friend once described it as "that generic Mario Party look."
If you removed the HUD, I would argue most people wouldn't be able to tell the difference between an HD emulator shot of NSMB Wii and NSMB U.
Meanwhile, go play Super Mario All-Stars. All four Mario games in that were brought up to the same baseline visual fidelity, and you can very easily tell them apart:
Versus...
One screenshot is from a game released in 2009, the other is from a game released in 2012. Can you tell which is which? Mario is using the same animations, maybe even the same character model. For reference, SMB3 was 1988 and SMW was 1990 - only two years separating them, and a noticeable change in art style.
And furthermore, you don't have this problem with the 3D Marios. Sunshine, Galaxy, and 3D World all appear distinct, even though by that point, Nintendo had established and locked down Mario's "visual identity".
That being said, NSMB U does seem to at least try to do more interesting things
with its visuals compared to past entries, but its roots as a bland-looking NSMB game are still pretty obvious.
But honestly, for me, the problem are the controls. SMB3 and SMW feel a lot tighter than NSMB ever did. People have said NSMB is trying to replicate the controls from SMB1, but I think it's kind of ridiculous to ignore almost a decade's worth of control improvements. That's being retro to the game's detriment and I can't stand it.
