The argument you are putting forth goes against the principle that the country's population can be trusted to make their own decisions as long as no-one else is harmed. Marijuana has a track record of being the safest drug around, safer even then alcohol.

Furthermore, the argument that terrorist groups benefit from the marijuana trade is just a myth. Most street-level cannabis dealers would get their product from someone who grows it in the same country - the profit margin of cannabis is small compared to other drugs so importing it is usually not financially worthwhile. Also, you're forgetting that ending the prohibition of marijuana would cut these suppliers out of the system and force them to go legit. Alcohol prohibition in the '30s forced alcohol into the hands of organised crime, and provided them with a source of income as well as putting the general population in danger of an unregulated alcohol market. Once the prohibition was lifted this took away the criminals' largest source of income and allowed the government to control the standards of alcoholic drinks. The same principle applies here - organised criminals would have one less source of income, and the rumoured 'dangerously strong strains of cannabis' could be outlawed so companies growing the plant would only sell it at a sensible and controlled strength.

Also, your argument about cannabis users experimenting with other drugs such as cocaine and heroin: this 'gateway drug' argument is simply unusable in the argument concerning legalisation because legalisation would remove the drug's gateway effects. The idea of cannabis being a gateway drug stems from the idea that, in pursuit of buying cannabis, an individual would associate themselves with the people who sell it: criminals. Drug dealers (like any businessman) naturally want to maximise their profit so they would offer up a stronger drug later on. The lack of distinction between cannabis and stronger drugs (because they're all illegal anyway) would mean the user is more open to trying these drugs. Legalisation would not only remove the criminal market but would also create a visible barrier between what is safe to try and what isn't.

In conclusion, the legalisation of marijuana would be, ultimately, a harm reduction measure. Gangs would lose a source of income and would become less powerful, users would no longer have to associate with criminals, and for once the population of what is meant to be a free country would have the power to make a decision that affects no-one but themselves. That isn't even touching on the other benefits of legalisation, including freeing up space in the over-crowded prison system, providing an additional source of government income in the form of tax and freeing up police time to tackle violent crimes.