First I do not know why it is a problem to transfer some of your profit to fiat,you do not want to pay tax or your crypto exchange does not have KYC?
I think that banks are perhaps the most powerful institutions in the world,and if they want it is very easy for them to strike on cryptocurrency very hard.Imagine that all or most of worlds banks say that buying/selling cryptocurrency becomes illegal,it means no direct money flow from banks to exchanges and vice versa.People would have to find another way to to buy/sell crypto,so only to trade in person or maybe on ATMs.
So far banks still evaluate situation what to do with BTC and cryptocurrency in general,we see some news that they are not still seriously concerned(not losing to much profit I suppose),but if profits star to go down and Lightning Network really improve scalability and make transaction fast&cheap there will be some reaction from banks for sure.
Regarding ICO I do not have any opinion how would such scenario reflected on this kind of business,but in general they will also be affected if banks are set completely negative toward cryptocurrency.
I am still heavily invested in cryptos, but if wanted to move money back to fiat, there would be problems. It's not about taxes at all. It's all about the AML policies of banks. For instance, assume I have my main account with Coinbase. They have strong enough KYC to be on the banks "approved list". Let's assume I wanted to withdraw a few hundred thousand USD.
1. If I transfer the amount all at once (assume for the moment Coinbase does not have any restrictions), this inflow in my bank would raise red flags: unusually large amount from a crypto exchange. The bank's compliance team would then require to see some evidence how such a large amount was obtained or generated. In my case. I am dealing with a multitude of exchanges and a large portfolio of crytos and I would not be able to trace back where exactly the gains would be coming from.
2. If I do it piecemeal, it's even worse. After a few smaller transactions from a cryptoexchange, the banks alarm AML alarm bells will definitely go off. And very possibly the account would be frozen immediately.
As for your other points, although banks can't declare cryptos illegal, if all banks would stop any crypto-related in / outflows, cryptos would surely die. As long as cryptos are not BROADLY accepted as money (including, for instance, in your local grocery store; or at your car dealer; etc.), they will have to be changed in fiat at some point. But if no banks accept the inflow of such fiat generated by cryptos, there would be no possibility to get to that fiat money for anyone.
The constraints above I have described, are already reality at some of the largest, global banks. So this is not just what might happen, but what is happening. I know that my bank has cancelled client accounts that were not able to provide the background to how they made the gains, even though it might have been completely legal. And a colleague of mine at another bank had to stop a very lucrative trading strategy he was running as the frequent money flows in and out raised too many red AML flags at his bank.