And this is where you hit the nail on the head but miss the mark....
Race/gender/whatever swapping characters to slot in the best actor for the part IS an absolutely reasonable idea that generally ends up with fan appreciation and acceptance (easiest example, Nick Fury). Nobody suspects that they cast Samuel L Jackson as Nick Fury to show off that they put a black guy in Avengers. They see it and go "its because they could get Samuel MFing Jackson to play the part".
However the fact that so many of the shows and movies that have done it recently end up casting actors who DO NOT do a good job, even a remotely good job, indicates that this factor has slid away from casting decisions (as has releases of casting calls that literally say what they are doing...) and have instead went with "cast a black guy for this role". They are no longer attempting to cast the best actor for a role, regardless of their race/gender/etc, but are now casting BECAUSE of it.
And while maybe this show is different, and they literally did it purely because of the amazing actors who they selected are going to kill those roles so hard...... the problem lies in that the same types of projects, for years now, have shown that not to be the case. Its much more likely they cast a subpar actor purely due to their desire to fill a "we need this many black guys, or gay women, or whatever rediculous requirement they came up with" and cast based on that, rather than ability.
And it shows.
And people have started to catch on.
And casting people like they have been, where their ability is secondary to their immutable characteristics isn't leading to good products.