Mt Gox is long gone, bankrupt, owners sued, although they still had a pretty huge stash of Bitcoin which they have paid fines years after the collapse, after the value went up. Everyone got fully compensated for their loss, tho in fiat so they missed out on their crypto gains.

Exchanges is, and will always be a weak spot, just like banks. This is why people say "not your keys, not your coins". Only keep on exchange what you can afford to lose, and don't keep it there for long.

There's always been ways to use Bitcoin, even without internet, or connectivity in general. Out of all systems online that can fail, Bitcoin will be the last thing to go offline. People have made transaction over satellite, CB radio, wifi, bluetooth, nfc and all kinds of alternative protocols. In a recent power outage I supplied power to the modem via a battery and managed to send and receive crypto that way, even tho DNS and all centralized services where unreachable.

But in general, there should always be offline alternatives. And that's the good thing about Bitcoin, because it's market valued it's perfect for barter with gold, silver or any other items, just like you would barter with gild, silver, bullets, cigarettes or eggs in a SHTF situation. It will help establish a barter economy. Fiat on the other hand could never do that. It'll just crash and then you'll be back using bullet,s gold, silver, cigarettes and eggs anyway, assuming the society isn't falling into complete chaos as people panics when they don't know what to do.