If Semien stays home, then it's a runner on third with two outs. If when they go after Semien, Bichette goes back to 2nd, it's a runner on 2nd with two outs.
But that would be dumb.
No. Going off the base to try to save Bichette was dumb,
as demonstrated by the play. All if did was get him out instead of the runner on 2nd. Which would you rather have? The runner on 2nd with two outs or the runner on 3rd?
By the time Semien made his move, two outs were nearly guaranteed. He just changed which baserunner it was going to be who was out. There was no advantage to his getting himself out so Bichette could try for third - Semien was already there.
And he had ~0 shot at home.