They're probably busting champagne if the peak valuation is instantly after IPO. That means they maximized the potential payoff.
You're right that one perspective to consider how well these companies are doing is to look at their continuous growth, and that includes post-IPO valuation.
At the same time, YC backed a couple of dozen unicorns. Including those that, despite having a valuation lower than at the time of their IPO, are still in the 10-digit range. Well, I'd be the last one to complain if I got to seed fund a company that ended up at $50B+ valuation.
Having said that, it's just a financial aspect of the argument, and it's only focused on post-IPO, whereas most YC-backed unicorns have not yet done so.
If you look at product innovation, you'll see Dropbox, Airbnb, Gitlab (save for Airbnb, all failures by your standards), Stripe, Deel, Zapier, all household names in my world. Why weren't these products developed by big techs?
You could easily point to one that would be especially interested in taking over that part of the pie.