I'll grant you that... I'm not into nostalgia radio and 1950s/1960s comedy acts. It's just not a big area of interest for me.
I have very little doubt that's true for most people today. If you weren't alive back then or didn't particularly pay attention that stuff probably went over your head.
This is NOT a slam on Stan Freberg or anybody else's remarks in this thread.
NOBODY can be interested in everything! C'mon that's impossible and there's not enough time in life for that! Nobody can listen to and watch everything!
Animation is a huge interest for me (mainly classical US from the late 1930s to mid-1950s, Disney classic features, and a lot of anime since the 1980s) and that's where I generally know voiceover artists and Mel Blanc was STILL by far the best-known voiceover artist during his lifetime...
and that's NOT just in animation. I've sampled a lot of Mel's work and
he was fantastic in just about everything he did! (I'm sure Stan Freberg was, too, but he wasn't as well-known as Mel Blanc was... NOBODY else was!) He was a great comedian with an impeccable sense of timing and evidently a VERY good memory. There was a Johnny Carson episode where he was guest along with Jack Benny. He remembered routines they did together 30-35 years ago! (This was in the early 1970s while Jack Benny was still... Benny was still 'together'/mentally in decent shape although his memory was not as good as Mel's.) Just about everybody that has even a minor connection/familiarity with animation knows Mel because he was around forever (around 60 years!) and he DID EVERYTHING and his projects tended to be listened to or seen by a bunch of people... He was on The Jack Benny Show fer cryin' out loud and that was VERY popular for decades on both radio and TV. And you can find quite a bit of Mel's other non-Looney Tunes work online now.
I have been to conventions, I have talked to voiceover artists, and I have been through a few seminars here and there just out of curiosity. I know these guys work their butts off to make a living and feed their families... and quite a few of them do actually seem to enjoy what they do very much, too! ( => unlike a lot of other working stiffs.)
I don't work in the industry but I know enough from being around people who ARE professional voiceover artists that they don't do just one thing. Most of it is radio/TV narration and advertising for radio/TV but quite a few DO crossover into animation but probably not as THE main gig in most cases; as I understand it, the animation voiceover field tends to be more restricted than other voiceover gigs -- there's only a few people that get to play regularly now. One of the worst things I think that happened in animation voiceover to the regulars was when celebrity voice-stunt casting became very popular starting in the 1990s... Some of the guys you hear more in shows like Futurama, Ren & Stimpy, etc. complained about that practice and I think to an extent they were right. What is the point of casting a celebrity when you're NOT going to see their real face onscreen? You might as well hire an impersonator! They would be cheaper than hiring the actual celebrity! (And you can spend more money making the animation better or actually developing a better storyline which is probably even more important than the animation!)

They didn't use to do that as much for animated feature films as they did for TV series like The Simpsons where it was 'cool' for a celebrity to do a cameo/guest shot (and they actually did feature 'Simpsonized caricatures' of those celebrities, too). The better-paying gigs I've been told are things like narrating movie trailers. I can't remember the names but I know there were people who made their names doing narration for trailers and you heard these guys FOR DECADES because they were considered the best at what they did. One of the legends of this died not too long ago... (Was it Don Fontaine?) I can't remember the guy's name but I am 100% SURE I DID hear his voice in trailers many, many times...