I honestly wouldn't read too much into the 8% figure. It's just a historical ballpark figure for minimum adequacy to weather all but the biggest shocks (in which case, it would be sensible and appropriate for the central banks to step in and play their lender-of-last resort role).
Most banks after their post-GFC workouts run much higher than this these days. Running 8% equity:CoreTier1 ratios these days would be seen by regulators and the market as "flashing some ankle", if not a full-on Janet Jackson "nipplegate". The effective minimum is well above 8% these days!
Plus the 8% is based on equally subjective risk-weightings for different kinds of loans. Back in the EUrozone Crisis, a bank owning PIIGS (even French at times) govvie bonds yielding 10% was Basel "safer" than a bank lending to German students at 4%. The market CDS of those Banks was obviously not consistent with the regulatory measures ;-)
What the CAR tries to guage is how much loans can go bad before the bank runs out of equity (rather than they cannot ever go bust thus). It's more of a "margin of safety" thing. How much equity do they have to cover potential losses?
Note that this does NOT mean that lending is immune to losses that the banks can cover without failing. Say I have 100 loans, 10 equity with 1 provisions booked for bad/doubtful debts (BDD) = 10% CAR. 2 loans go bad, and I recover 25% = 1.5 losses. Less the 1 provision for these already booked, that's a 0.5 hit to profits and equity. So I now have 98 loans, 9.5 equity, 0 BDD = 9.7% CAR. But I now have no provisions for BDDs on the rest of my book. Provisioning for the same 1% of assets, that's a 0.98 hit to profits and equity. So I end up 98 loans, 8.52 equity with 0.98 provisions = 8.7% CAR.
I'm not bust, or even "underwater" from a regulatory perspective - but my capital cushion has been wiped out if I expected 1% of loans to sour and 2% did. So I will curtail my lending (even start calling in riskier debts) long before my solvency ever becomes a pertinent question. This is the real (and still unresolved) issue with banks!
hope this helps