>You cannot stop a p2p decentralized open source network
Pass a new law: said p2p decentralized open source network is now illegal.
Network stopped in 1 day.
You seem to underestimate the power of the governements.
Very few people are ready to go to jail in order to keep using the bitcoin.
I'm not saying that this a likely scenario of course.
Just because a law is passed saying something is illegal doesn't mean the network will stop. If only we could pass a law making drugs illegal, the drug markets would shut down overnight. If only we could pass a law making tax evasion illegal, then the millions of people working under the table for cash would open up bank accounts and reenter the taxed economy overnight. If only we passed a law saying p2p decentralized open source bittorrent networks are now illegal, then digital piracy would stop overnight.
In case you can't tell, I'm being sarcastic to illustrate the absurdity of your conjecture. You are overestimating the power and ability of governments. The main way the government has to enforce laws is voluntary compliance by the general public and ridiculous laws don't get compliance from the general public and thus is too expensive and difficult to enforce. Laws against things like rape, theft and murder work because in general people agree with them and voluntarily comply, so enforcement is just on a small percentage of outlaws, not against the general public.
In the incredibly ignorant event that some government makes bitcoin illegal, they are destroying their own power to regulate and/or profit from it it. Since governments thrive on power, wealth and control--exercising power and control over bitcoin to extract wealth is their likely strategy over simply making it illegal. Regulating and taxing bitcoin makes the government richer and thus more powerful as well as give them an additional domain to exercise bureaucratic control in, killing bitcoin makes the government poorer and thus decreasing their power and depriving them of domain for them to exercise their bureaucratic control in. If you want to predict the future behavior of governments, just imagine what action would give the government bureaucrats the most power, control and wealth and this is the action they will take. Making bitcoin illegal doesn't increase the power or wealth of the bureaucracy as much as regulating it does.
Also such a bitcoin illegality scheme would have to be worldwide to kill bitcoin and there are countries in the world where the legal regime is such that this would be unconstitutional. For example, in the USA, we have constitutional protections on freedom of speech, and it can be argued that bitcoin is speech. If I send you one of my bitcoins, what I am sending you is nothing but a string of numbers, which can be verbally told to you or written down on a piece of paper. Even transmitted over the bitcoin network communication it does not lose constitutional protection as "speech" simply because it is expressed in the language of computers, just like if I reply to you in French it is just as protected as if I reply to you in English. So making bitcoin illegal outright would violate my constitutionally protected rights of freedom of speech and freedom of association (with other bitcoin network users). As long as there is at least one country in the world that still recognizes freedom of speech, bitcoin will still exist there.