What determines whether a bubble pops or not is not logic but rather belief

Well... there's harder economic realities that people eventually bump into. And that shapes belief in the credit markets and the investment banks.

For instance, Greenspan hiking interest rates from 2006 to 2008 was what put Lehman Brothers and Bear Sterns under pressure to cover an escalating interest payment. When they began to default, all their creditors panicked (justifiably) and began withholding shorter term lending. And that accelerated the collapse. But because of the network of interbank lending, Lehman tugged on the strings of Goldman, WellsFargo, BoA, JP Morgan...

It wasn't just vibes. There was a material credit crisis resulting in Bear Sterns - specifically - not having enough money in the till to pay its credit notes. And because everyone was so highly leveraged, there was a real accountable cascade of defaults.

In theory, bigger institutions could have backstopped the slide by offering even more generous credit terms (which is what the Federal Reserve / US Treasury ultimately did). The "vibes" part was the risk/reward analysis. Everyone could see the looming risk. Relatively few executives could conceive of the long-term consequences of a collective bank run.