The problem is given gerrymandering Democrats probably need to be winning by double digits or at least very high single digits. If Democrats win the popular vote by say 7.5% they probably fall short of winning the House.
There's a point with gerrymandering where the dam breaks and losses end up worse than they would be otherwise. This is an entirely illustrative example, but if you gerrymander every district to be R+5-10 and there's an across the board swing of D+12 then you get wiped out despite getting say 44% of the popular vote.
