No, 2017 counts, but it is not the only statistical data point that should be considered.
You cannot look at 2017 in isolation, the same way that it is not fair to look at one good year in isolation.
Davante Adams started 11 games as a rookie and played in all 16. You know who also started 11 games as a rookie?
Even if you take him at his career average of 10.1 ypc, he's still 90th. I have been very open to taking his career as a whole.
Landry was a rookie in 2014. 9 ypc. Can kinda throw it out the window because of his youth and inexperience. But then he put up 8.8 this year. Those are the 2 most similar seasons he has.
2015 was better 10.5 ypc, closest to his career average.
2016 was Tannehill's best year. So, I'll give you maybe if Tannehill was healthy, they would have been able to continue that with their rapport together.
But that's still 1 out of 4 years. Shit happens in the NFL. There's no guarantee that Trubisky would even be as good as Tannehill was in 2016.
But overall, I think the big issue with Landry is that a throw to him is a throw not going to anyone else. It's not his fault per se, but I don't want 161 targets to my slot WR. Maybe that's a design flaw in the Dolphins offense (but it was essentially the same design before Gase). Maybe it's not. But when you take a normal pass target distribution, Landry should only get 90-110 targets. At that point, you're looking at a 70 catch, 800 yard type of WR (that also typically doesn't score much outside of 2017). Not nearly worth the money. And if you make him earn the money by throwing to him 160 times, you are making your team worse because you aren't taking huge chunks of yardage available on the outside.