If the voice actors are still in well shape, then I can understand their disappointment, but no matter what their voices have obviously evolved somewhat by now. I'm not sure if they would be able to continue to replicate the same voices for the entire series, so it was pretty obvious that they were going to be replaced, now that you mention it.
If the new VAs do the cartoon justice, no problem. I'm wary of giving this show a reboot since it's a big classic for the kids of the late 90s and earlier 00s (including me), but if it does a good enough job, I have no problem with it, though it would be nice if the voice actors could have a cameo in one of the episodes.