After that review, Zille said that they will be content with being a 20% party and that the focus would be on retaining the "core electorate" i.e white people. That is the reason i am skeptical of anything which predicts significant growth for the DA and believe that they will come out with between 19%-22% of the vote.
Their focus for the last few years has been on bringing back stability and the trust of the core electorate (which they seem to have done to their credit), and not on growth outside the base as was their aim which failed under MM.
They will have a choice to make after JS ends. If they want to maintain the path of stability and play it safe, they could go with Hill-Lewis as leader though I don't see him making inroads outside their traditional base but he will probably keep the base happy. If they want growth, they could go with Pappas as the next leader though you could argue he does not yet have the experience to be leader so it might be a bit of a risk at this point.
Pappas was a former COPE member so he could have some more progressive views which some of the traditional base may not like as well. National leadership is a totally different ball game to regional. If MM stayed within DA Gauteng, he probably would still be popular within the party today.