Remington didn't do it because they wanted to be PC, they did it to be CHEAP.
The new assembly was an Express "thing", to allow Remington to offer an 870 at a price that would be competive with Mossberg and Winchester.
In addition to the usual Express cost-lowering changes the plastic plug system is just plain cheaper to manufacture than the Wingmaster original system.
Most people don't notice the more expensive to manufacture Wingmaster magazine cap and ball detent assemblies. These are just complicated enough to raise costs, where every cent counts.
It costs time and money to pay a worker to insert a spring and ball into the barrel ring, and stake it in place. The cap is machined with milled detents.
It costs less to use a robot to punch in two lugs, and to cast a plastic plug. The cap is simpler, with "formed" teeth inside.
This all costs a few cents less. That's about all it's about.