His production very much warrants his paycheck. Since 2015 he has the 14th most TD's in the NFL (24), despite being 38th (313) in targets when going by all WR's who were targeted at least 300 times since 2015. His pace of a TD catch every 13 targets is better than DeAndre Hopkins who scores a TD every 17 targets, Antonio Brown's a TD every 15 targets, Devante Adams TD evert 14 targets, Odell Beckham's TD every 15 targets. Obviously he's not as a good as the gentleman I just referenced but Still's catches TD's at a good rate is the point.
The fact that he is 14th in TD's even though a million balls went to Landry, he played with Jay Cutler, and most recently a broken Tannehill/Osweiler is a testament to how valuable he is to the team. Also since 2015 he is 7th in the NFL with TD's of 20+ yards with 16. Only Brown, Hopkins, Cooper, Hill, Baldwin and Cooks have more. For 8 millions per year I would say he's worth every penny for the production he brings to the table.