I think we are forgetting that Sony has a number of other metrics that we don't have in addition to reviews and sales. Such as average playtime, completion rates etc.
So in addition to the need for discounts to move that number of sales, its entirely possible that a good number of people who bought the game didn't end up playing it for a significant amount of time.
In this particular scenario I think forcing the studio to move on to a new IP was the right decision. I think Sony could have allowed the sequel but in all honesty its likely that the Days Gone fanbase is limited and if the sequel is just a continuation of story with slightly added gameplay elements then I think the opportunity for growth is near impossible. This is perfectly fine but it needs to be weighed against the estimated cost it will take to build the sequel.
Investing in a new IP is a clean slate for the team and Sony and the potential fanbase for the new IP will most likely exceed that of the Days Gone sequel.