Well, there are some nuances you're skating over here.
Remember that GMAC Bank (the direct banking company) and GMAC ResCap (the mortgage/real-estate company) were two different entities owned by the industrial loan company GMAC. GMAC Bank was spun off as Ally Bank. Ally Bank (until 2014) was not even in the mortgage business.
I can tell you that as an actual customer of (the now former) GMAC Bank, I didn't feel like I got short changed in any way. They had good interest rates, and I'm glad they survived through hook, crook, and some TARP money (which was ultimately paid back with interest).
Just my perspective, it doesn't affect me in any way who you bank with.
https://ally.com/