That I'd agree with. I'm not saying Let it go wasn't apart of its success, just not the sole reason. (I'd argue that Let it go is pretty much what makes up half if not more of the Home Video sales, though.)
<.< I also like your Welcome as much as I liked Let it Go.
Am I the only one who thinks boiling down Frozen to a song does it a massive disservice? It also played as a deconstruction of Disney Princess movies, which helped it's popularity with older audiences. Let it Go may have appealed to children, but it's play on true love is why adults were willing to see it in droves as well.
Spielberg: Jurassic Park, Schindler's List, Indiana Jones and the Last Crusade
Hitchcock: Trouble with Harry, Rear Window, Dial M for Murder
Kurosawa: Rashomon, Yojimbo, Seven Samurai