If the valuation starts off so high that means future growth has already been priced in, which means there's no compelling reason for any new investor to buy the shares. The share price will drift downward for years (which happens to most IPO's) unless they can completely blow the doors off with new revenue.
It's just my anecdotal experience. It's pretty easy to prove the IPO market lately hasn't been like it was in the 90's and that's the reason the big tech companies have been staying private.
