Would secession ruin the USA? Hard to know. Secession from the EU has certainly been bad for the UK, but the rest of the EU seems to be doing ok.
Note that we heard before Brexit that it would not cause problems, yet it in fact bring benefits as expected by anyone with any idea how economy works.
UK has a 6% decline of the economy due to Brexit
The UK might have paid the biggest price due to Brexit, but the EU also lost a significant trading partner for it's goods and services. Arguably it was a lose-lose outcome.