This is 100% correct, it's exactly what I've thought for a long time. Like you have put Koeman was to pig headed to give him a try and only did in the end due to desperation, Sam started off here much the same by not really involving him much, but now it's finally clicked that Niasse helps the team much more than he thought, also Davies does much the Same. From the off this season, if Niasse and Davies had have played instead of DCL and Snides we would have a fair few more points, but recently the addition of Walcott has also added much more to our attacks especially pace.
I like DCL and hope he improves on his weaknesses.
I agree though:
Davies, Niasse more involved from the beginning of the season would have certainly made a difference. We used to think that Niasse could only be an impact sub, but that was down to his match fitness. Importantly, Niasse, Davies and I would say Gana (without Schneiderlin) and now Walcott, , add a lot of energy, determination and drive to the team, and you surely can't help but look at them and feel shamed into running, trying hard and working for the team...this attitude must be infectious
Pickford, if he were an outfield player, would demonstrate the same kind of energy
Would add Rooney as well, if only for his presence if not his fitness and energy , he at least looks and behaves like he cares