Ubi always do this, they forget an open world game needs content post-completion. AC could do it because they used the idea of the animus but for this series I think they couldn't work out how to integrate it. Far Cry 3 eventually got a patch that allowed outposts to be repopulated so hopefully they'll release a similar content update for this game as its definitely needed. It wouldn't even be that hard thematically, have a laptop in each hideout that has the 'CCTV' footage from each mission and allows Aiden to review it (replaying the mission) giving the option of using all current weapons which the game would then scale up the difficulty for appropriately or a set-up the game chooses at random (ie what you likely would have had at that stage). There could also be a smart phone app of similar function on Aiden's phone.
I hope they do sort this out or I might not bother buying their stuff as I'm a big story person so to lock it out especially as there is only one save file just makes no sense.