If someone cracked the hash algorithm, or found a cheaper way to compute it, they'd make a lot of money, difficulty would go to the moon, but the rate of Bitcoin creation would not increase. Just like when ASICs came in.
Think way beyond that... if you can.
For lack of a better example: my gripe is that this is a bit like Windows 98 security: You can set a login password and it gets encrypted satisfactorily... but to bypass the login password, you simply needed to press ESC at the right time. So, you think, if you press the right buttons, you don't need to sign a transaction with your private key?
That exactly shows, that you have no idea what you are talking about.
Not at all.
"For lack of a better example" <- Did you see that, what did you think it means?
Almost like "pressing the right buttons" - I can transact using your wallet because I stole your private key the moment you generated it, so I can sign any transaction with your key. (The NSA is my side project)... I'm still accumulating keys. (And metadata on others, which is almost as good as the real thing, consdering the amount of computing power I commad...) How many keys do you think I have, by now?
See... you have no clue.
I also have backdoors installed at all the major exchanges and wallet services and I'm best friends with their cold wallet keyholders and know all about their routine, protocols and families... Oh, I almost forgot, I also have a backdoor in everyone running a client or server compiled from the github code, ready to fire it up when they send their next packet to another node... (LOL that code has been in there since v4, and nobody has spotted it yet) Also, I have a virtually unlimited budget, and have just been doing this "for fun"... you know, just in case you tried something funny, just because "Uncle Bob" told me to keep an eye on you.
So I'm keeping my finger on the trigger. If I press ENTER, everyone who co-operated with me gets filthy rich... and Bitcoin dies. (But if I do it now, something else will just take its place... just...have...to...wait..a...little...longer.... then nobody will trust crypto ever again. Yes! Like communism.)
You still have a chance to stop me...
You have nothing, no numbers, nothing, except a poor excuse: "Hey man, SHA256 is really good. Look it up. We're safe." ...Do you know how many people I talked to? Do you know how many keys I have? Do you know how much of the network I control? Do you know my endgame?
Do you want to see what a hostile takeover of the world's biggest crypto currency looks like, before you believe that it's possible?
Being able to "attack" a system because you have a password/private key and really attacking it, are 2 complete separate things. If you are too stupid to understand that, than I can not help you.
Your scenarios are based on "I have infiltrated everything. I have even magically infiltrated you air-gap machines". If you could really do that, why bother with bitcoins? Just get access to all nuclear silos and threaten to bomb everyone.