Counterpoints:
1. He had a year left on his deal and was looking to leave. If he went back, he'd be locked in for multiple years.
Yes, the contract situation lowered the price, but we sold him at the top of his form and would get him back at a low point. To put it bluntly, Kagawa is a shadow of his former self and this is also down to himself. There is no telling if he ever reaches his top form again.
2. Gundogan's injuries could open up a spot for Kagawa, with them playing more of a 4-1-4-1 than a 4-2-3-1 with 2 of Jojic/Kagawa/Mikhtaryan playing in front of Sahin/Bender, or sometimems both of the latter and just one of the former.
The 4-1-4-1 has become a viable option for us and is basically the offensive counterpart to the 4-3-3 we began to play in the CL in 2012. Both these formations are situational, though, and only played against specific opposition. The core system is still the 4-2-3-1 and this won´t change in the future according to Klopp himself.
3. Klopp has spoken many times of Kagawa's ability to get goals from midfield and with Lewandowski going, another goalscorer might look good to him if Dortmund are struggling to score enough, especially if Reus demands to leave and gets his way.
We have no shortage in terms of goal scorers. We have four offensive midfielders (Reus, Mkhitaryan, Aubameyang, Kuba) with enough goal threat to score at least 10+ goals per league season and assist around the same amount. Reus won´t leave this summer. We brought in another one with Ji, who is very highly rated by Klopp. Furthermore we replaced Lewandowski with a clear cut finisher and a target man similar to the Pole (albeit with less quality outside heading).
If anything we gained goal threat in this window.
4. Dortmund have 2 very good CB's in Sokratis and Subotic, they just bought a top young one in Ginter and they have another young one, Sarr, I think, who they seem to rate highly.
None of them could fill the void Hummels would leave behind right now. While some people overrate Kagawa´s former influence at Dortmund, others also underrate Hummels in that regard. He is on and besides the pitch our most important player. Period.
There is also no one on the transfer market right now who could outright replace him no matter how much money we invest. The only chance to compensate a potential departure of him in the future is to develop someone towards that role. This is the whole idea behind the Ginter transfer. Ginter is a similar player type and has enough potential to become as good as Hummels one day but he is not there yet.
As long as we don´t have someone ready to take over Hummels´ role in the team, we won´t sell him out of our own will. Dangling a player in a well stocked area in front of our nose won´t change that. If Hummels leaves in the forseeable future, it will be because he really wants to leave and not because we want to sell. The only scenario of Hummels leaving right now would be if he forces himself out of his contract. They are absolutely no indications for that, though and Hummels would take a massive hit in terms of reputation (something that was always important to him) to do it now four weeks before the season starts. Based on how he came across his own career and positioned himself in the public, the chances for that are basically zero.
In general, I though, agree with you. Dortmund have good CM and ACM depth and Hummels is a proper star. If they bring in anyone it should be a right-sided version of Reus, or at least the closes they can find since those don't grow on trees. Even if Hummels goes, I think they'd be better off with a ton of cash to go buy Griezmann, Shaqiri (though I guess that can't happen), Lucas Moura, Cuadrado, Draxler or some other wide goalscorer.
I don´t see us bringing in another wide player, especially someone of the mould of Reus. As good as Reus is (world class and our best offensive player), he functions the best with freedom to roam around. We already have another player who thrives on movement in Mkhitaryan and an additional one would be overkill and take up the spaces the two other need. We actually have a pretty nice balance in the offensive midfield right now with the two mentioned roamer, two well balanced midfielder (Kuba, Hofmann), an AM/CM hybrid (Jojic), a fast counter player (Aubameyang), a devensive orientated option (Großkreutz) and two striker with the ability to play the CAM role (Ji, Ramos).
There is no need to invest further there as long as nothing changes. We will adress the Reus position when we´ll need to replace him, so most likely 2015.