I actually think the opposite.
Mitchell was a good top up for a side that has some of those star midfielders that just need an additional legup. The sort of final piece of the jigsaw if you like.
We weren't in that position, we needed the underbelly filled in and we have done that and this is where free agency comes in. You as well as anyone on here know that a premiership midfield, needs top 5-10 midfielders, Hopper and Taranto provide that, and by staying in the finals mix it helps us attract quality free agents so that we can continue with the rebuild on the run.
Your thinking IMO is outdated. Its exactly the way teams would have rebuilt 10-15 years ago. Go through a period of being up, knowing you will have 5 or so years on the doldrums. We are trying to avoid that, by maintaining quality and then adding with quality free agents. Going towards the bottom, restricts you to the free agent quality of players like Karl Amon who will not take you to a premiership. Free agency is different if you are at the top to the bottom, the top you are adding that final quality in a key area of deficiency to take you that final step (see Tom Lynch). Free agency at the bottom of the ladder, you get in some quality to support your young youth as you compete at the bottom of the ladder.