With the new currencies for upgrades, all the milestone rewards that used to be minor (some green papers, mining resources etc) are now important items required for research. But players who already hit these milestones cannot receive these rewards. So older players who return to the game have none of the items required for research, and a major way of obtaining them (through these milestone rewards) is unavailable for people who have grinded out those (less important) rewards before the update.

Perhaps a one time, optional milestone rewards reset could fix this? Even if it's removed from the game later it would still give currently active veterans the ability to catch back up to where they were on the research tree by grinding out each level again.