The Raptors going balls deep for Davis is a no brainer.
Let's say it takes Siakim, Wright, and a future 1st, just for arguments sake. Hefty price that impacts the future. No doubt. However, Davis still has another year on his deal after this one. So this season plays out. Maybe the Raps win (then it's all worth it), or maybe the don't...in which case, you turn your efforts to resigning Kawhi. If he walks, then you can make the call to pull the trigger on the whole thing and start a rebuild. At that point you just re-deal Davis to Boston, LA, whoever...and basically recoup what you gave up in the first place. Teams will still be lining up to trade for him and his value really wouldn't have gone down much...if at all.
The only question is, will giving up Siakim, Delon, and/or whatever it will take from the current roster, impact their potential success this season by decimating their depth? That's a different discussion though.