As you see it, if (hypothetically) an industry lasts for five years, and then collapses due to none of its promises coming to fruition, was it in fact a bubble the whole time? Would people have been wrong for calling it out the whole time, even if it took five years to happen?
Whether or not this describes Silicon Valley is a further question, but it sounds like based on your standard, you wouldn't give credit to a naysayer even if it did.
If hypothetically, the entire industry collapsed, sure, the people who called it a bubble were right. But an "industry collapse" means something like Google or Facebook shutting down with no competitor to replace it, almost every unicorn going broke, and most importantly the market cap of the entire sector hitting pre-2011 levels proving the growth was fake all along. That would be a really big collapse - to the extent that pointing to a bad Yelp earnings as evidence is a joke. In real bubbles, big, established players like Bear Stearns and Lehman Brothers disappear and thousands of smaller shops get crushed. A collapse is not a few unicorns losing some value. Even if Uber got wiped out I wouldn't call it an industry collapse, and that scenario is already hard to imagine.
More likely, at some point we'll see a bad year or two, some hiring freezes and layoffs here and there, and people point to that as a bubble collapse that they "knew all along", even though they "knew" it during times when the sector grew more than it eventually receded.
In real bubbles, you see a insane amount of growth based on pure speculation, followed by a massive collapse. The justification for a "bubble" so far has been based on incredibly silly things like the engineers making 80% of what lawyers make or private valuations being high (which only affects a small number of people and is often inflated by things like liquidation preference incentivizing it to sound artificially high despite a significantly lower risk).
Companies like Zynga, Groupon, Living Social, Homejoy, Twitter and Yahoo struggling during a so-called "bubble" is not insane growth, it's proof the market has maintained skepticism over the years. And a few more unicorns falling over won't be a massive collapse either.
Every market has downturns. Not every downturn is a bubble. And an actual "bubble" existence hasn't been proven true despite repeated insistences for 5 years, and I highly doubt it will start now, unless we uncover massive, massive fraud.
Reply threshold. Saying a bubble bursting always involves a Lehman Brothers or Bear Stearns collapse (brother post) is pushing it. That was a leveraged bubble and it burst harder than most. In the dot com bubble most of the big names rode it out from AOL to Yahoo and Amazon. A bubble can pop without a Facebook or Google type company going down.
Whether or not this describes Silicon Valley is a further question, but it sounds like based on your standard, you wouldn't give credit to a naysayer even if it did.