Cabrera and Fielder did not play their entire careers for one team. They moved on during their most expensive years. So from a Blue Jays fan's perspective, if Guerrero is destined to have 30 WAR here no matter what, we should be pretty indifferent to whether he achieves 45 WAR or 0 WAR elsewhere.
Now if you have some attachment for him as a person, or want to see another Blue Jay in the HOF in 2045 if you plan on living that long, then these things do matter.