In a magazine interview about OW2 they said that their will be progression systems tied to these replayable missions. The talent/skill customization system we saw at Blizzcon (leveling up heroes to customize and upgrade their abilities and ults) is just one of several systems they have planned, according to them.
Basically the incentive for these, from my understanding, is to level up your hero and increase their power and customize their playstyle to be a completely unique hero from everyone else’s version. And then I’m assuming you can increase the difficulty, etc.
I also read the endgame content is being worked on by Diablo 3 and WoW head devs so that means it should be pretty good! Those guys know how to do PvE!