Who wants to change the source code to suit the needs of regulators?
Wouldn't large miners (who basically control the network) have to comply with any provision issued to them (including modified code with whatever rules regulators think of to code in there)? Regulators don't bother with this just yet, as they see that Bitcoin is too small and fading away by itself to become irrelevant due to its flaws, and it's cheaper for them socially to just wait it out. But if it didn't fade away and began to grow to threaten the fiat system, be sure, they'd issue this license in a heart beat.
When it is a change, that changes how the blockchain works, you would need a hard fork. Users/miners would just have to stay on their old version to not accept it.
Alright, users and small miners would stay on their old fork, but large miners that control the network would have to comply with the regulations or shut down operations. Eventually users would have to either sell their Bitcoins (sell them on the old or new fork) or switch to and stay on the new regulated fork as well. What's wrong/impossible with this picture I just drew?
You have the incentives wrong. Your mistake is starting with the premise that miners control the network. They don't. The economic majority controls the network. If the economic majority doesn't agree with a change, it won't happen (or last for long).
Let's imagine a scenario where a large percentage of the global hash rate wants to implement a change which a large percentage of the economic interests do not want.
This group of miners forks Bitcoin. Everyone who has bitcoins at this point now have coins on two chains. They don't want the new forked coins, so they immediately start selling them. The price of the coins on the fork starts to plummet (a majority is selling). If those sellers happen to find buyers for their forked coins, they will now have additional funds which they might use to purchase actual bitcoins. There will be massive (we are assuming an economic majority here) downward pressure on the price of the forked coins, and possible upward pressure on the price of actual bitcoins.
You are a miner. You have costs. You may even be in debt for hardware.
Are you going to mine the forked coins which are rapidly decreasing in value or bitcoins which are at worst holding at the same value and possibly even increasing in value?This scenario can of course go the opposite way, if the economic majority wants the change, they will liquidate their actual bitcoins and possibly use the proceeds to purchase additional newly forked coins.
The point is, the miners are at the whim of those who value the coin. If they try to force changes on the economic majority, they will quickly find themselves mining worthless coins. Prohibition can not stop economic forces (typically quite the opposite). If mining is outlawed, then outlaws will mine. If a coin is valued, people will find a way to mine it, regardless of any legal hurdles.
If you're a large mining farm, you're going to mine whatever fork the regulations dictate, if regulations force you to go bankrupt, well, then you will go bankrupt. That's the nature of large businesses, they are easily targeted and regulated. Bitcoin is just too small now, so they don't bother, the whole market cap of Bitcoin is pocket change to them at this point, it's politically not worth raising hell about. The economic majority may as well support the new regulated fork, and the network will keep humming along. But will this new fork be any different from fiat?
