I don't think that altcoins are overpriced as you said.
Bitcoin dominance at this rate is not a healthy indicator for the market. It would be healthy to my eyes if I see a respective increase in altoins' prices instead f just bitcoin's.
Why?
Almost all of them have achieved precisely fuck all since their peak a year and a half ago. No adoption, no use, nothing new. That's mainly down to the bear market of course but that still gives time to build and develop and not many have made use of that time.
An ever increasing amount of big money is sniffing around Bitcoin with more financial instruments arriving as well.
I could just about understand why people got excited about a few alts in the heat of 2017. In the cold light of 2019 why would you make the same bet again for most of them?