I think the concern is that kind of player is the best we're going to get here, at least under Belichick.
A 75 catch/(close to) 1000 yard WR who's not even the focal point of the offense with two other main players in front of him would be just fine for any other team, but once those players go down or miss any time the "real" LaFell shows up and we're screwed. As you already know, Llyod was another who was "good enough" under normal circumstances, but disappeared once his role shifted.
I didn't like passing on Sanders the first time with that meaningless, lowball offer, and when they failed to bring him in a second time I was even angrier. If we're going to upgrade the receiving corps then it will likely have to be by getting rid of both LaFell and Amendola, and even then we'll still have to find the right fit who's going to accept that 5m dollar AAV deal.
(I'm tired and it's late, so my apologies for any data or math errors below. If there are any, they are not deliberate.)
For the better part of a decade, this team has spent its highest capital on the defense. Since 2006*, the Patriots have devoted the top of their drafts to the defense. The following list is a year-by-year list of the defensive players taken prior to an offensive pick being taken.
2007 - Meriweather/Brown
2008 - Mayo/Wheatley/Crable
2009 - Chung/Brace/Butler
2010 - McCourty
2011 - exception: Solder was chosen first
2012 - Jones/Hightower/Wilson/Bequette/Ebner/Dennard
2013 - Collins
2014 - Easley
2015 - Brown/Richards/Grissom/Flowers
Looking at another angle, here are the picks in the top 3 rounds, where a team is most likely to get players:
2007 - 1 defense, 0 offense
2008 - 3 defense, 0 offense
2009 - 4 defense, 2 offense
2010 - 3 defense, 2 offense
2011 - 1 defense, 2 offense, 1 QB (A frickin' QB drafted in the top 3 rounds in 2011)
2012 - 4 defense, 0 offense
2013 - 3 defense, 1 offense
2014 - 1 defense, 1 offense, 1 QB
2015 - 3 defense, 0 offense
Total high pick investments, 2007 - 2015: 23 defense, 8 offense, 2 QBs
They also haven't spent any real money (I'm talking comparative, not absolute dollars) on any offensive players in free agency, in quite a while, with the arguable exception of Amendola, whom they got because they wanted to low ball Welker rather than pay him, and instead paid the lesser salary to DA when Welker balked. In fact, they traded away their best OL because they wanted to go on the cheap. They got away with it one year, and it bit them on the ass, hard, the next. And, as you noted, they tried to go cheap on Sanders, and lost him to the Broncos. Hell, they could have gone low money and grabbed Incognito, but they didn't even do that.
That all needs to change. It's past time to invest more in the offense. And, with the 1st round pick gone and high-end trades a rarity in the NFL, that's probably going to mean a focus on offensive free agents, if they're looking to get things done in 2016.
* The first round pick split from 2001-2006 was 4/3 offense, and there was no 1st rounder in 2000.