Chose the right regression analysis
Since I cannot comment because I don't have enough reputation, I will post this as an answer.

If your goal is to "find a point where the increase of promotion don't increase profit or new_users", I won't do a simple regression, since the regression will tell you that if you do more promotions, you will always inscrease the profits. I would say that, in reality, the relationship between promotions and profits or new users is not linear. Because the number of new users is limited and the promotions are not.

A better model is to say that there is a optimal promotion that will give you the best increase of profits and new users.

(if you have a real business to optimise, I would introduce the Customer Lifetime Value of new users. Because generally, the new users you get when doing huge promotions will not come back...)