Lindor giving 15 fWAR over the next 5 years is fine, but you’re talking about ages 33-37. It’s very possible you only get 8-10 fWAR in those years, especially if injuries factor in.
Money is not the issue, because Lindor would cost lesser prospects that could equalize the costs.
But Lindor is getting into that age range where decline can be sudden and without warning.
And then there’s the question about why are the Mets trying to move him? Cohen has a greater net worth than the bottom 10 MLB owners combined. Is their real goal strictly financial?