Collie was actually brought in to replace Boyce not Thompkins. Josh Boyce was a complete waste of space out there with a grand total of 5 targets through the first month of the season. There was no choice but to make him inactive.
We've seen our share of WR's who don't play their first season, as the team uses it as a red-shirted aspect so that they can take their time learning a complicated system. They likely either figure that we're good at the moment with shorter/intermediate receivers (Amendola, Edelman, recently Collie), and/or that Boyce just wasn't picking up the system well enough or establishing a chemistry with Tom Brady. Maybe a combination of both.
I don't think it has much to do with where anyone was drafted, as we've seen UDFA's start over high round draft picks plenty of times; although I think we can all understand what your point was with the statement. I'm sure Boyce will be back in the mix next year and should see much more playing time.