Thing is... Sentinels/Marauders have slightly more survivability than a shadow in 23/1/17 spec, and more damage capability as well. The only things that 23/1/17 spec has the advantage in when compared to a sentinel is they are slightly better in 1v1s, and they have more utility than sentinels do.
Any good team is going to have a 23/1/17 shadow either guard their node, or try to delay/1v1 the enemy at their node rather than deathmatching with the other dps. Do you know why? Other damage classes are better in a group setting. Sentinel is better in a group fight while still having more survivability and damage. I'd suggest to nerf them first.
You can't defend a class by saying another is more overpowered. It just doesn't work.