Matt Chapman last 3 years:
28 (oakland) - 100ops+ 3.5 bWAR, 101 wRC+ 4.1 fWAR
29 (toronto) - 116ops+ 3.5 bWAR, 118 wRC+ 4.2 fWAR
30 (toronto) - 108ops+ 4.4 bWAR, 110wRC+ 3.5 fWAR
It seems to me that his bat is not the reason for him being a 4 win player, it is his glove. Chapman plays a premier defensive position elitely and that is how he racks up WAR. We have seen this with multiple players who are bad with the stick but great with the glove that play in premier positions (Grichuk, Varsho, Pillar, Simmons, etc.).
I think the question is, do you think as Chapman gets on the wrong side of 30, that a) his glove holds up b)his bat stays league average? If not, that contact (20+) would be an instant albatross.
BTW: IKF over the last 3 years: 1.6 fWAR, 1.5 fWAR, 0.2 fWAR - for shits and giggles: 3.6 bWAR, 3 bWAR, 0 bWAR. You can make an argument that the Yankees yanked him around too much to pick up defensive WAR, and by putting him at his best position (just heard on the broadcast that he won a gold glove at 3rd for Texas) he should revert back to the 1.5 fWAR/ 3 bWAR guy he was just 2 years ago.
If they get that production, isn't 7.5 a steal and the type of upside you are asking for?