It seems to me that you haven't watched him beyond one season (even that I'm not sure of as you're only mentioning stats). Plus you're singling him out in one particular bad game he had. Not an objective analysis.
Anybody who watched 2015/16 knows Blind is a very good passer with even better vision. Since Mourinho's arrival, he's had to restrict his forward passing, esp while playing CB. To a minor extent, our present game doesn't bring out the best in him but even then, I'd argue he's our best left-back.
Actually, I am from the Netherlands and have seen Blind play for many years. At Ajax, on loan to Groningen and in our national team. In our country, he is known to be a player who passes the ball around to the sides and to the back alot, while not producing many great forward passes. That does not mean he does not create great passes every now and then, but in general, it's not that special. The difference between Blind and Carrick in their passing as CDM was hugely visable. If someone plays it safe in their passing alot, it's Blind.
I am nog singling out a bad game, as people say he played well past sunday. As people tend to praise his passing ability and say he did great in the season opener, I do not think it's unfair to mention the difference in passing rates between our LB and RB in that match. A single game is nothing to take conclusions from, but a difference of 21% is alot. As a LB in an easy game with plenty of room to go forward, he should have done better on the ball. For someone with a supposedly great pass, it should have been easy to have whipped great crosses in, but most of them never got passed the first defender. This should be a thing he should excel in, compensating for the defensive weaknesses, but I just don't see it.
Bottom line in my opinion is that Blind is a very decent passer of the ball, but nowhere near the quality to be able to play in midfield at United. Defensively, his game has some serious flaws in it and that is something we can not overlook. This has been made painfully clear in plenty of matches, especially against stronger opponents. It's no coincidence that Blind got dropped from the starting 11 after about 10 games into the Premier League last season, only to be played against weaker opponents from there on. For instance, just search google for 'chelsea united 1 0 pedro'. It just hurts your eyes to see what he is doing. There are several examples of situations like that.
His passing style is more a playmaking style, and in my opinion that's not so valuable for the LB position. His crossing is, in my opinion, not that good either. Taking into regards that he has been taking corners and quite a few set pieces in the past few seasons, the amount of assists he produced with all those efforts is mediocre at best.
So, then comes the question if you should keep playing a player that is weak defensively and also not that great of a offensive threat as a LB. I would rather see a more talented player come in who can actually develop into a great LB for years to come, rather than sticking to 'meh' options like Darmian and Blind. In the case of Blind even more so since his versatility to play in several positions declined in value with the purchase of Lindelof, the upcoming of Tuanzebe and also Matic (for as far as Mourinho even saw Blind as an option for CDM).
I do not by any means think Blind is a worthless player. I just think he does not have the quality to be playing for Manchester United. Ofcourse he has and probably will still put in some good or maybe even great performances. Overal it is not good enough and his flaws are just to big for me to neglect. If we can only play him against weaker opponents, which Mourinho has done, bring in someone like Kieran Tierney who is young, shows a lot of talent going forward and might develop into a class player for us.