They will never totally shut down one of their game because the economic backfire will be too big: think of all the money people has put in HotS, if they close the game servers everything will be lost but then even players on other Blizzard’s IP will start thinking if their game will be next so microtransaction will almost stop because people don’t want to dump money on something that can be taken down.
HotS will be put on mantainance mode (so no one will “lose” his money), probably at the start of 2020 when all the already (or almost ready) to go Heroes that are already stored will be released. Then active development will probably stop and the game will be kept around with just balance patches here and there.