Why a Malzahar or Rengar Revert is very unlikely
The only reason riot reworked Kog'Maw, is because his very existence breaks all on-hits as nerfs to him really wouldn't affect the situation. The ONLY possible solution for Kog'Maw was a revert. It's sad because he was actually kinda fun and had some counterplay, but IMO every reason that riot gave to his revert is a lie. This was the only reason. As for Malzahar, if they can find a state where he is kinda ok, they won't revert him. Same for Rengar. As long as he's not Kalista'd, then he won't get reverted. Even in the event of a champion getting gutted, a revert is probably still unlikely
TL;DR As long as Malzahar is viable as a support, there will be no revert. Period. As long as Rengar is viable as a bruiser, there will be no revert. Period.