If Lenny now time slipped back to the early discussions when they were rolling out the counter and the referral program, would he then vehemently oppose the ideas, citing all that he's learned? "Duolingo isn't like a match-3 game" "Language learners aren't referring discounted rides"
Maybe he would? Maybe he should, because he's learned these lessons. But was younger Lenny's job to be the whippersnapper who says with conviction "you don't know if you don't try, let's give it a shot!"
And so in a sense you must have these two types of people, and once you've been through the gauntlet, you either make it, and have been right all along, or you become wise, and your job is now to be the naysayer.
Or, if you're an experienced PM, are you instead thinking, this isn't perfect hindsight, but simply a largely avoidable detour before he arrived at the metrics that mattered? Maybe a critique of the actual journey is even more educational.