I think the problem is the old 80 to 60 drop is now 80 to 65 with the crappier refrigerants, thus requiring more airflow to transport the same amount of heat.
It’s weird they only want to change the return ducts (negative pressure) and not the supply ducts (positive pressure). Maybe they are worried about the return duct collapsing because of lower pressure near the furnace (higher airflow and velocity in same duct = higher pressure loss = reduced static pressure near furnace).
They didn’t explain anything. Off to other companies for competing bids.
They'd have to really spin the blower up for that to be an issue, don't you think?
Anyhow, good luck.