I don't know about that; the moment those coins are suspected to be compromised, you'd get a near-immediate run on BTC. So yes, there are a lot of Bitcoins available, but stealing them would undermine the entire system their value is based on, rendering them worthless.
They wouldn't be rendered worthless, but the security would take a hit for sure. Any coins mined (and not moved) before 2012 would be at risk, in addition to any address that has been reused since then (revealing the pubkey, instead of just the pubkey hash). Addresses created post-2012 that have not been spent from (spending reveals the pubkey) would still be 100% secure.
Eh -- "secure-ish" at best. Spending those utxo's requires revealing the pubkey, at which point someone with a quantum computer can just break your pubkey and offer a new transaction with a higher fee while your transaction is still in the mempool.
