I can't believe this is actually happening! I wish Charles the best in his life going forward and hope Nintendo finds a good replacement down the line. His voice as Mario was so distinct and iconic. And it also fit in with the rest of the Mario cast quite well. It wasn't brash like Lou Albano or Walker Boone, nor was it a Brooklyn accent like Bob Hoskins or Chris Pratt.
This probably means that Luigi, Wario, Waluigi, and the Baby Bros. will have to be replaced as well, since they were also voiced by him. Luigi's voice is distinct, unlike Tony Rosato's, Danny Wells' and John Leguizamo's takes on him. Charlie Day did a decent Luigi voice, but Charles Martinet gave depth to the character by making him sound anxious and worried at times.
Wario and Waluigi's voices are unique as well, they sound tough and tricky, like they might be up to something. The Baby Bros sound very cute and adorable, so I hope they aren't butchered by whoever replaces Charles. (Please don't let their replacements be young kids, Nintendo. It won't work out in the long run if you do that.)