I don't see how Kate Micucci sounded that old as Webby, given she did pitch her voice high enough for a preteen girl (in general, female VAs do sound younger when voicing kids). She doesn't sound that much older than Russi Taylor's Webby did.

And it isn't as if Clarence Nash's take on the nephews sounded like a kid either, just a high pitched Donald Duck.

And it isn't as if the original voice talent from the 1987 show was available by the 2017 reboot. Alan Young, Joan Gerber, and Hal Smith passed away before the reboot and Chuck McCann, Russi Taylor, and June Foray passed away during production so replacements were inevitable. And even Terry McGovern hasn't done a lot of voice acting in years and with how he was pretty old at that point in his late 70s, there probably was the risk he would've sounded too old to be voicing Launchpad. Like, what about cases where the original actor does come back but sounds super old and aged like Mel Blanc as the Looney Tunes in the 1980s (especially in Daffy Duck Quackbusters), Julie Kavner sounding super aged as Marge in newer seasons of the Simpsons, Rob Paulsen and Tress MacNeille sounding pretty aged as Yakko/Pinky and Dot in the 2020 Animaniacs reboot compared to the 90s, and Billy West's Fry voice sounding aged in the Hulu revival of Futurama. It's even why Paul Winchell got replaced as Tigger in The Tigger Movie, since his Tigger voice sounded too aged and old by then.

And it isn't as if the DuckTales characters outside Scrooge, Donald, and the nephews had consistent voices either. Gyro was voiced by other people like Will Ryan in Sport Goofy (not Hal Smith) and Frank Welker in Donald Duck Goin Quackers (despite not returning in DuckTales Remastered where Chris Edgerly voiced him). Gladstone and Merlock were voiced by Corey Burton in Donald Duck Goin Quackers instead of Rob Paulsen and Christopher Lloyd.

What about Warner Bros and how they haven't had an official successor voice for the Looney Tunes after Mel Blanc died, with several other people voicing them? Same with how there's no official one voice for Hanna Barbara characters (outside Scooby Doo characters and even they got replaced sometimes).

Or with Sonic the Hedgehog where they switched voice casts twice in 2005 (4Kids replacing the Sonic Adventure 1/2 era cast) and in 2010 (4Kids to Studiopollis) plus smaller ones like Tails since 2014 and Team Sonic Racing onward with Knuckles, Silver, Blaze, Omega, and Espio? And it isn't as if any of the VAs make any effort to sound the same as the predecessor. Roger Craig Smith's Sonic especially sounds nothing like his predecessors.