We're actually doing it. 29 states medicinal. Eight states legalized.
In 2008 when Barney Frank and Ron Paul proposed the Medical Marijuana Patient Protection Act, there were only 13 medicinal states.
Wikipedia has this great gif showing the collapsing prohibition in action:
https://upload.wikimedia.org/wikipedia/commons/3/36/Cannabis...
https://en.wikipedia.org/wiki/Timeline_of_cannabis_laws_in_t...
That was all unthinkable just as recently as 1992 when Clinton was elected on a tough on crime platform. Then we watched the prison population number go from 800,000 to 1.3 million just in his eight years; the greatest increase of any President or eight year period since the mass incarceration effort began in the mid 1970s.
Now we've got the Republican President standing down (compare that to Nixon, Reagan or the Bushes), and the top Senate Democrat arguing in favor of Federal legalization. We've come a very long ways in a relatively short amount of time.
What this signals, is that with continued pressure and legalization in more states, Federal legalization is a done deal. Whenever the next Democrat wave arrives that gives them enough votes, that will formally complete it. Otherwise, it's going to get legalized by most states regardless (which will realistically guarantee a Federal legalization).