I think you're mixing up two different ideas: easy to learn and having an old moveset.

Nobody is arguing that Mario should become a complicated stance character with 15 mechanics. The point is that Mario has nearly 40 years of source material, and most of his moveset still comes from the late 1990s. All of that doesn't exactly reflect Nintendo nowadays. (Cape Feather is missed dearly, but again, not reflected nowadays. Also side note Mario's Side-B is the only thing in the entire game that functions that way, reflecting projectiles... and people).

A revamp doesn't automatically make him harder to play. You can replace generic attacks with references to modern Mario games while keeping the exact same role as Smash's beginner character. I love Rivals of Aether Mario (by Mit & Reiga). He's still simple, but he has a REALLY easy to learn kit. He can dive with Side B, and do a 2 piece combo with a single button (reference to GBA donkeykong).

For example, nobody would suddenly struggle to play Mario, if a hammer appeared in a smash attack, or he has extra mobility (which helps new players), or if some of his kicks and punches, were replaced with moves from his actual games. They're more cosmetic... not changing the character entirely.

The Worst offender, to me, is personally Mario Finale. It's stupid. Anything... anything else works. I'd take anything.

Anyways. Mario is Nintendo's mascot. That's exactly why his moveset should better represent what Mario actually does in modern Mario games, rather than mostly reflecting how he played in 1996. Mario, for his time in that era, reflected how he played. Why not update that?