Williams only leaves in a trade, IMO. His salary for 2018 is guaranteed, and he's a 5 year starter, with better overall stats than Hurns. Nevertheless, their stats are similar.
Williams - 5 years, 80 games, 230 receptions, 3,359 yards, 20 TD's - averages per game - 2.9 rec, 42 yards, 14.6 ypc, 0.25 TD's
Hurns - 4 years, 52 games, 189 receptions, 2669 yards, 21 TD's - averages per game - 3.6 rec, 51 yard, 14.1 ypc, 0.4 TD's
But, Hurns has missed 12 games in 4 seasons, and Williams has never missed a game. Williams is also a much, much better downfield blocker, which is important in the Cowboys scheme. Don't let the fact that Hurns is a shiny new toy distract from how reliable Williams has been for the Cowboys.
Having said that, Hurns is more of a hands catcher than Williams. Adding both Thompson and Hurns may indicate that Sanjay Lal is having some influence on the team. It is possible that the Cowboys are going to revamp their pasing offense this year, and maybe Thompson and Hurns are better fits for what the Cowboys will try to do under the guidance of Lal.
I hope these signings remove the possibility of drafting a receiver in the 1st round, which would be an awful choice.