People worry just too much about that 7th D situation.
History has proven that you very rarely get your optimal Top6 on the ice. There's always injuries, there's back to back games for rotation etc.
I once counted an average, how many games Red Wings 7th guy played (years 2005-2015) and it was something like 57 games on average.
Also, 8th D usually had played 15-20 games, so there's one 82-game "guy" missing from Top6 through the year.
This situation will be sort out when they'll start playing games. Someone will get hurt. We don't know who. But some will. There will be pre-season groin problems etc. We see that happening every year.
BUT, when you have extra depth to replace this unnamed guy having an injury, your season won't get f***ed up for that. Our team level does not change anywhere if 1 guy is missing, with the current deep D-core.
If you don't have this depth, your season will blow up for injuries. This depth is needed because they want to reach the playoffs. It's a sign of real team.