Any currency can be used illegal purposes. For instance, there are many offshore accounts to do money laundering.
That doesnt matter at all, bitcoin can resist EVERYTHING. Im 100% sure of that. Programmers can do anything and goverments won't win ever.
Because bitcoin can be maintanted on offshore land, where u can't regulate it.
But Bitcoin needs to improve a bit to survive and rise now... As more people start joining Bitcoin and are doing simultaneously transactions the speed is decreasing a lot at a point many transaction won't be completed or will take too much time to be completed, it can make the investors lose the interest, taking more time to rise Bitcoin among the world population.
I don't know what you understand by speed but for me any transaction that is done during 10-30 minutes is fast enough, faster than any other payment methods and can be withdrawn with the fastest speed through any exchange.
I too agree that Bitcoin can resist until the end of time without a single problem in it's way.
Many people are facing too long transactions, taking too much time to complete, not 10-30 minutes or even some hours, but more than 24, 36 hours... This always happen when there are many people doing their transactions at same time.
So, as you can think, when a lot of people around the world start using BTCs the fee will be too high if you want to have your transaction completed, if you pay a low fee probably you won't have your transaction completed. It can make some people lose interest in Bitcoins...