So lets assume that the financial crisis of 2008 happens again. Would 401Ks / ROTH IRA account holders be at risk of losing everything?
Who else might be at risk to lose everything if the Government bails out the biggest banks again due to the ineptitude?
No, they would not be safe. BUT...it depends on what your investments are in within those accounts. For example, in the 2008 market crash (in the US) Citigroup went from $50 per share to $3 per share. If your 401k was invested in Citigroup at that time then the balance of your 401k account went down just as significantly as the value of those investments.
Now, if your 401k investments are in a cash investment only, which is covered by FDIC (Federal Depositor's Insurance Corporation) AND the bank with which your 401k account fails, then your FDIC insured investments would be reimbursed up to $250,000 per account.
The accounts you name are tax-sheltered accounts, that is they protect you from how your are taxed on the investments or the gains. They don't inherently protect you from anything else.
I share your contempt for government bailouts, but the bailouts are offered primarily to save investors from the fall out of poor business management.