I think you have the Superpets all wrong. The Streaky the cat wasn't from Krypton, so it was just a normal cat most of the time. Comet the horse, was really a man, Supergirl even dated him and married him once! Keep the jokes to yourselves, human aren't pets, and he was not from Krypton. Beppo spent most of his time in Space and was rarely ever in any comics. He was from Krypton. It actually makes sense to test a rocket with a monkey or ape rather than a dog, since that's what we humans did in real life!
I know the entire history of the super-pets already, but thank you for restating it for those not familiar with it. Anyway, whether they were from Krypton or not wasn't really a factor to me. Frankly, I like Kryptonian survivors--it allows for much more stories. Krypto was obviously a cool character (especially in Maggin's
Starwinds Howl), but the other pets never really did it for me.
Comet would've been more interesting if he were actually a centaur (wasn't there a story where he was?) and as for Beppo, well, let's just say that I prefer Titano to him. But I do agree with you that it makes much more sense to send a primate into space rather than a dog. I suppose Streaky was okay since he didn't have his powers all the time. Still, I prefer only one super-pet--Krypto.
Lots of Iron Age fans try to make it seem that these types of stories happen all the time, but truth is they were very rare, and you can collect them all with little problem, except for Krypto who appeared in a lot of tales, that's why people actually know and like him. You hardly ever got to see Streaky or Comet unless you read Supergirl. Beppo was just in Superboy for the most part. These animals hardly appear in the actual Superman comics at all.
They only appeared together a handful of times and that was in The Legion of Super-heroes Comic!
The reason that those fans say that is because they're promoting an agenda. They want to turn people away from the past in a
Brave New World-esque way. In short, they look down on the past and don't want anyone to enjoy it.