These fans have recognisable talent, and their video makes for a fun what-if experiment, but I'd never seriously argue this direction fits in a movie. Especially when it comes to Toad. Fully agreeing with Ray Trace on this one.
There are two reasons I can think of for why fans want the original voice actors so badly: 1) inherent inability to accept change, and 2) they're not fond of celebrity casting. Neither of which are particularly sound reasons.
Though, I concede Mario's French voice actor lends a wonderful performance, one certainly better than Pratt's--not due to the inane, perceived obligation of having the character sound like "muh squeaky Charles Martinet tenor" as much as for the fact that it's hearty, in line with the film's up-beat atmosphere, while still sounding natural, like an uncle who's popular with his nephews and nieces.
I totally agree about Toad's constant yelling, it can be too much! However, with Mario's voice range able to reach a soft pitch for somber/serious scenes, cut back on the "yahoos", while still keeping the voice, I think it can be done. Cartoony voices are fitting for a cartoon movie, no? The issue wouldn't be the cartooniness for a cartoon movie, but the vocal range of emotion from serious to silly I'd imagine. For instance Disney's Goofy has a cartoony voice, but can still perform a wide range of emotions from sad scenes, to angry scenes, to silly excited scenes.
Resistance to change isn't completely without sound reason, as change for the sake of change isn't necessarily good. A good change could be an Italian American actor like Danny DeVito being cast to do an Italian American close-to-NYC accent for Mario, it's a change that sounds
very different, but still faithful to Mario's origins. A bad change could be casting Clint Eastwood, who is so old now he would just sound like an old man.
Change is a balance, bring in some new for excitement, but hold on to enough of the old so the identity isn't lost making the adaptation entirely pointless. If people are concerned as they feel Chris Pratt's performance isn't preserving enough of the old (Italian American Plumber roots, can be anywhere from Martinet-style to a New Yorker) I think that's fair.
Wariness of celerity casting has grounds too, not because there aren't any compatible celebrities, but because the process of celebrity casting doesn't usually target compatibility. It's not uncommon for movies to pick celebrities to increase sales off of name recognition, over merit for the role. I won't make any accusations here, Chris Pratt could turn out amazing in the final version for all we know, but the precedence for skepticism stands.