Different players, different needs.
We already have our own version of Cooper Kupp, named Tyler Lockett. Sadly, he was unavailable vs the Rams and he's Russell's #1 security blanket.
We're better off having DK on our team than Kupp, assuming Lockett comes back healthy. We'd underutilize Kupp anyway, unless Waldron's scheme and Russell's ability to excute it really caught fire in year 2.
However, a physical talent like DK is harder to find. If only we could do a Dr. Frankenstein and transplant Kupp's maturity and football savvy into DK, now that would be special.
DK stretches the field for us
DK is a better blocker in the running game, and is a willing blocker
There were two or three instances in the Rams game where it seemed like DK could have made the catch on a deep throw with a little more craftiness, to set up Ramsey, instead of ego-driven trying to just out-athlete him. Those would be advanced skills though. Does DK have the drive to learn and master those and add them to his game?
If Kupp was the same age as DK, it would be worth considering. They're not. So DK potentially has more high-value seasons left than Kupp.