It'll fall, but I don't think it'll fall that much. I agree, it'll be interesting to see what happens. Either way, it would have to fall A LOT to drop below where the 1.10 likely goes in startups.
Let's keep in mind that Marshall is only getting $5M for this year and has no guaranteed money in 2018 (and it's only a 2 yr contract). He got paid less than Wheaton, Garcon, Stills, Britt, and Woods did and is getting the same this year as Torrey Smith. In other words, this isn't an expensive deal and I don't even think it's fair to assume yet he's even the #2 WR there.
In other words, from a dynasty standpoint, the addition of Marshall is a small blip on the value of a 24 yr old dynasty WR. Again, for the same reason D. Henry isn't and shouldn't be discounted much just because D. Murray is coming back, it shouldn't change his value that much (even though we'll see if it does).
ETA -- Here's one last tidbit on Shepard and then I'll leave it alone so we don't turn this into a SS thread.