"If net fifteen Republican seats are taken by the Democrats, they have the majority."
Yep, you are right.
But taking into consideration redistricting overall and the areas that were stuck in court battles and are now resolved , I think even with the bad feelings floating around for some Republicans it would be impossible for the Democrats to take back the house.
The Senate, that is another story. I could maybe see that happening , but still unlikely in the next election.
The country has been pretty well dissected by the parties and there is little room for either party to grow in significant numbers anymore.