His career average matters little to me; he's not in Miami. And he missed games in his time in Denver. I'm talking about targets per game. Look at his targets/game with Cutler - 2012 was simply more of the same. And if 29 is old - might as well start shopping Calvin. Then Julio in a couple years.
In an average season you usually only have a handful of guys with 150+ targets and maybe one or two guys above 170. Marshall is one of the best receivers in the league and remains a top redraft option, but I'd bet good money on his targets dropping next season. I've already provided the reasons. Martellus Bennett, Jeffery, and whatever rookies they might add between now and the start of the season will cut into his looks. That could be offset to some extent if Chicago passes more now that they're out of the ultra conservative Lovie Smith era, but even super elite WRs like Andre, Calvin, Fitzgerald, and Marshall can't reasonably be expected to get more than about 150-160 targets per year. When they eclipse that, it's usually because the stars aligned in their favor.
That's not really the point though. I don't doubt that Marshall is one of the top short term receivers in the NFL. The issue is his decaying trade value.
At 29, he's already reached an age where certain kinds of owners won't have any interest in paying a fair price to acquire him. That will only get worse with each passing year. Over the next 2-3 years his value will drop steadily while Julio will maintain approximately the exact same value that he has today. It might be a good deal if all you're looking at is the value they'll give you on the field for the next couple seasons, but when it comes time to cash out there will be a radical difference in what you can get for Marshall/Jennings/Davis and what you'll be able to get for Julio.