I wish it could be backwards looking, but I completely understand your reasoning on why it shouldn't be and would support that. I'm obviously extremely biased here because of all the referrals my videos have generated, so I'll mostly step aside on this debate and accept whatever the team/community wants there.
Is it ok to simply leave this up for the team to decide, or should I edit the proposal to specifically state this?
You can leave it up to the team to decide. I certainly understand your bias, and in an ideal world I would prefer to have it be backward-looking too, but the unfortunate reality is that I think it would just give people who have exploited the system in the past even more rewards.