Let’s not kid ourselves, pothead types wouldn’t become Republicans over decriminalization. They would be thankful for 15 minutes and then go back to their normal habits.
You are also right about human nature. What have you done for me “today” is the mentality but that is true for most people, not just libtards. Timing matters in such situations.
You make the biggest political gains not with those you already have and will support you anyhow, rather those that you don't have. You win a war when on the offense, not in a defense.
When you do something that is consistent, logical, ethical, may have a net economic benefit and NEVER EVER had the stated intent to counter drugs or crime in the first place, you win, even if you only make marginal gains among folks that traditionally are not your voters.