Oh come on.. polymer, synthetic, composite, "military grade glass-reinforced co-polymer" etc are just fancy ways of saying plastic while trying to avoid the associated stigma
I'm open to the idea that in some applications plastics can be as durable - or possibly even
more durable - than metals.
But let's not kid ourselves: the plastic trigger housing on the 870 is 100% intended as a cost-saving measure by Remington.
Either way I've never heard of anyone wearing out an 870 triggergroup through normal use, whether metal or plastic - and I wouldn't bother 'upgrading' either.
The basic 870 design has been around for nearly 3/4 of a century and (notwithstanding recent fit & finish complaints), it's the most popular shotgun of all time. If it had any
serious issues requiring aftermarket parts they'd have been addressed decades ago.