I don't know anything about Ruger's plans for Future semi-autos.
That being said, it is probably safe to assume that future pistol designs will lean towards polymer frame.
It comes down to cost of production. I have seen the numbers from a couple of development projects for other manufacturers. An alloy frame can cost 3x-4x what a polymer frame costs.
For an example look at what happened to the sales of S&W metal framed pistols once they were $100-$200 more than the polymer guns.
On the positive side, Ruger probably has a lower price differential since they are casting the frames.
For my part, I hope they keep making metal frames as long as they sell!
Thanks,
Kent