This is wrong on both counts.
It "might" help you win this year. The chances of that beyond this year go down. One down year or injury to either and value starts to really tank, let alone production. There are no guarantees that this deal helps you win when you otherwise wouldn't when you are dealing with 30 yo WRs. Recent examples of DT, Dez, Jordy (all very talented and very productive... until they weren't). Assuming both guys give you 14-15 PPG for the next 3 years is a bet I'll take against every day.
As for Guice, that's also misguided. He doesn't have to be quite good for a long stretch to be a trade you regret. In fact, he only needs to be good for a few games to regret it. See Kareem Hunt last year. Or Dalvin Cook, who even suffered a bad injury and his value is still retained. Or Mixon, who did almost nothing and still has huge value. Or Kamara who exploded.
And yes, I know there's no guarantee he turns into any of those guys, but the idea that he has to be good for a long time to be worth it simply isn't true.
If you can find a WR on the wire to give you 11-12 PPG, then you can likely succeed with Guice even being mediocre as a rookie if he shows anything at all. Guys like Robby Anderson or Chris Hogan or M. Sanu can give you that. The days of AJG or Baldwin scoring 18+ are very likely over (both at 14 last year). They are no longer difference makers.
Far more ways to hurt your team when you invest in depreciating assets like 30 year old WRs, especially before the season, than help it.