The only way to incentivize older players to do old content again would require a reward that is worth the time. But yet also it can't be spammable. It couldn't be a single piece of gear that is best in slot for everyone. IE the one that fits this bill right now is the nayame set from bumba, it benefits all players if path b is selected.

It would likely need to be something like "helper points" and there is a limit to how many you can get a month, resetting each month upon update, much like monthly deeds. And said points would be used to buy items of worth.