But Rooney doesn't even play at the moment. He is 5th in the list of forwards in terms of minutes played, the list goes like so...
Ibra,
Mata,
Rashford,
Martial,
Rooney,
Mkhi,
I would argue that the emergence of Rashford and the signings of Mkhi and Martial have actually replaced Rooney in terms of the positions he plays in. Though admittedly not adequately enough when compared to Rooney's "at-peak" goal scoring contribution to the team. So in fact what you are advocating is to replace one of our existing forwards because they have not contributed enough.
Now, I agree that they have not contributed enough. But in my personal opinion there's grounds to give them a chance one more year. Martial and Rashford are young and suffering 2nd season syndrome, Mkhi's new to league and was out of the team for a long period anyway. If it's the same story next year with the forwards, then bring in Griezmann or another forward by all means.
Like I said, if Martial/Mata/Mkhi bang in 10+ goals each next season, then there won't be a need for a new support striker. I do acknowledge that Ibra has been our best striker and that I'm advocating for changing our best attacker while keeping the ones that have underperformed. But like I said, we will have to change Ibra anyway if not this year then the next, but if our other forwards step up then they might not need to be replaced at all.
He's our biggest earner in terms of weekly wages and occupying the position (whether he is playing or not) as someone who is being paid as one of our marquee players. Griezmann would merely replace him in that regard.