I still remember how long it took for me to get used to Eric doing Frank's characters (and David doing Cookie), but now it's gotten to a point that I'm so used to his performances that it's almost disorienting to hear Frank's voices coming from those characters. Stephanie has been great as Prairie so far, and I cannot stress how seamless Eric has been as Oscar (and that was one voice I didn't think anyone could reproduce as well as Caroll), but Jen still needs to work on Zoe. Nevertheless, I agree with KarWash, the voices aren't going to be something inheritor performers are going to be able to completely imitate or reproduce spot-on, so you pretty just have to accept when characters' voices sound different due to different performers - much like Looney Tunes, look how many times those characters have gone through different voice actors (and still continue to do so) since the passing of Mel Blanc; some voice actors have done well with Bugs Bunny (Jeff Bergman, Billy West), others (Greg Burson) not so much. As great as the late Joe Alaskey was as a voice actor in general, I felt his Tweety was too deep for the character and made it seem like Tweety had gone through puberty of something (Bob Bergen did a better job at hitting that higher and more younger-sounding register), but at least he was able to really nail Tweety's speech impediment, mannerisms, and inflections.
Still, we just have to accept these things. If SST continues even more years down the line, who knows, there may come a time when, say, Carmen will have to pass Rosita on to another performer, or Marty with Telly and Snuffy.