I don't think Semien will be impossible to replace.
Maybe I'm not very good at figuring this stuff out, but a full season from Springer and Kirk could potentially be enough to replace Semien's WAR.
A bounce back season from a healthy Biggio likely adds another 3+ WAR as well, never mind if the Jays actually went out and signed or traded for players like Ramirez, Story or Correa.
For me, getting Ray re-signed is more important. It completely solidifies the Jays rotation 1 thru 4 (Ray, Ryu, Berrios, Manoah) and then you have guys like Stripling, Pearson and Hatch battling to take the 5th spot, and that's if the Jays don't dig up another hidden gem from out of nowhere like the seem to be able to do on a whim. That's how they got Ray, Matz and Stripling to begin with. Find and fix seems to be their forte. Hell, maybe they use their track record to lure Thor back to TO on a one year show me deal and then he comes in and is a CY contender next season.