If you think that now that GW stops buying tin the price of tin will go down, I think you've misjudged on how much tin is used outside of the miniature games industry. What GW 'consumes' is a drop in the bucket (maybe even the sea), so it will not impact the price of tin.
As for IWM going plastic, CGL is already doing that (starter set, 2 Clan Mechs, the Clan Mech set that has been planned). Maybe IWM will eventually have to go with resin or a resinplastic (more flexible) when tin prices continue to rise, I doubt that they'll sell many BT figures when they'll be forced to ask $50 for a BT figure.
Going to plastic with a core Mech/vehicle line could be very profitable for CGL, but it would depend on how they would package such a product, making a single mold for each Mech would not be a smart idea. Anything that isn't mainstream/popular could be done in low volume in resin.