Sure, and even legalizing all drugs in the U.S. and the Americas won't fix the problem, since cartels have other sources of income we will likely never legalize: human trafficking, extortion, etc. The full solution will involve painful long term reforms to the political and economical institutions of the countries involved (at least I know it will for Mexico), solving centuries long issues with structural income inequality[1], access to opportunities, race and class divisions, etc, etc.

But we are talking about closing a big revenue stream for the cartels, almost certainly one of the two or three biggest they have, maybe even the largest, just by lifting the prohibition on marijuana. That's not only a step in the right direction, it is a rather big one. In the long term, treating drug addiction as a medical / consumer protection issue in the US rather than primarily a law enforcement one, will be a huge boon towards the future of both nations (and, again, other nations in the Americas that I am not excluding but for which I feel less qualified to speak about).

In short: it won't magically fix the problem, but I do believe it will likely reduce the number of deaths and put us in a path towards a solution.

[1] For the HN libertarian crowd, I don't mean solving income inequality in the sense that everybody should make the same regardless of economic output (although I do believe on a safety net), I mean changing a centuries long setup in which class mobility is near zero and strongly correlated with race, and where most large scale "entrepreneurship" in the country is based of political connections (see telecommunications and media industries in Mexico for example).

Because it is a step. Just a small step but there is some hope that it could lead to decriminalization of all drug use.

Demand. There's nowhere near the demand.