When you have a player like Rashford, there is absolutely no reason to buy someone like Werner. I'l list some of the reasons below:
1) Rashford is as good a striker, if not better, than Werner:
If you want to talk about Werner of the 16/17 season, when he scored 21 goals (7 assists) in 31 matches, then I can agree that Werner is better than Rashford. If you look at Werner since that season, he has scored just 27 goals in 58 games (13 assists), that Werner is no better than the current Rashford we have, who has shown tremendous potential since Ole has come in (along with his first season under LvG). Had Werner continued his goalscoring form from the 16/17 season onwards, I would consider him being above Rashford, but, given how his form has dropped in the last two seasons (not only for RB Leipzeg but also for Germany) I wouldn't make that same assessment now.
2) Werner is not effective as a winger:
Some people here have compared Werner to Lingard, saying he can play as an effective winger for us. I don't think he is a world class winger and if he is played there he will be nothing more than a stop-gap solution that won't improve the first team by a lot, atleast not enough to make us firm title contendors. We should be looking for more natural wingers for the RW/LW position if we really want to make an impact next season, not people like Werner whose natural position isn't there.
3) We already have a great backup striker in either Lukaku or Rashford:
Depending on your preference, either Lukaku or Rashford will be the starting striker leaving the other on the bench. With that, we have two very good strikers already in the squad and if you want to play a mostly 1 striker formation, you don't need more than that. Werner won't be any better than either of them so it would be a waste of financies that can be used elsewhere.
Frankly, I think the striker position is the least of problems for United looking towards next season. Therefore, using 30-40 mil of our financies for a player of that position would be a waste.