This post is wrong on multiple levels so its little wonder you are puzzled.
Reinhart was most definitely starting to fade before he was sent down. Poor decision making and his skating was an obvious liability. He needed more development.
As for Davidson...he was never a write off. If that were true the team wouldn't have been at all worried about losing him on waivers. They protected him knowing there was a player there.
They were right....there was a player there. A player that (albeit older) is a better skater than Reinhart and is more physical.
I would even suggest that he thinks the game as well as Reinhart and makes better decisions.
BD is a keeper and I would be happy if Reinhart turns out to be as good.
I like that you tuck in that Davidson is older in brackets like it doesn't matter. Have you always thought Brandon Davidson was a keeper? Brandon Davidson played his 20 year old season in the WHL as an over-age player and now has taken 4 pro years before he's established himself as even a mediocre bottom pairing d-man which you'd be happy if Reinhart turned out as good.
Does Reinhart not get the same benefit of development that Davidson got?
I've never liked the trade at any point, and still don't. But even I can see how incredibly biased you are on this particular player. I'm just not quite sure why.