A player contracted for 2 years is a completely different story to a player with one year to go on their contract to the club. Look at the Boyd situation. GWS folded pretty quickly because the potential compensation they could get in that particular trade period was substantial, whereas in the next trade period he was likely to walk for zilch. Plus with two years a club can feel they have a reasonably comfortable buffer in which to try and persuade the player to stay, whereas one season feels a lot more taut.
I'd imagine Melbourne would fight for Hogan regardless, but if McCarthy stays with GWS this year and he still feels the same way next year, we could virtually halve or third the price we'd pay for him now.