The global economy actually consists of people doing useful things. Shuffling paper around can facilitate useful work, but it's not indispensible. Humans are inventive and would soon find workarounds if the big banks failed. The world would keep turning.
Well of course, the world can't crash, I mean even if it did it would restart its own economy, may it be carrots and donkeys rather than USD.
Yes... Money is only paper, debt. Tangibles remain. Of course insolvent financial entities should be allowed to collapse, whatever their size.
(schemes to insure small deposits are another matter)
Many speculative booms of the past, in which huge amounts of 'money' were lost, created ground-breaking technology or infrastructure which remained for good and posterity (Railways, Radio, Internet etc..)
The real purpose of 'Investment' vehicles like the stockmarket is to recycle idle funds (savings) into risk projects. A lot of the money is destroyed, but useful things are created.