To be fair, Sonic has been recast with multiple different voice actors several times over the course for several different roles, from various games to, the various TV shows and the live-action movie series alike.
Mario has only had one voice actor since like, he ever had a voice, period, not counting the cartoons or the CD-I games, and has been the voice of the character for nearly 30 years. The man's incredibly famous and he almost certainly made Mario even more popular than he already was with his voice work. Changing an iconic and beloved voice actor after that long,, regardless of reason, is gonna be something a lot of people, even if not everyone, will have to adjust to, so I do kinda get why Nintendo's holding off on revealing it and trying to make it a surprise.
Regardless of that, and whether or not someone needs to adjust to the new VA, there's ultimately still a lot of people who want to be genuinely surprised by the reveal.