In this case what you say is true, but with Milan's current condition they are actually quite cornered in my opinion especially with their financial condition which makes this a little difficult.
Milan's current condition in the standings is still very good but on the other hand we know that they currently have a lot of gaps that make them a little insecure and even after the break is over I'm actually still quite pessimistic about how they will face this season.
Napoli's consistent performance and Juventus recent form is surely matter of concern for the current champions and second position holder in league table because now if they want to stay at this rank or increase their chance for the defence of their title then surely they need to invest in coming winter transfer window and bring few quality players which are essential for them without them, they can do down because Juventus is already at his best with Lazio and Inter are also doing good work, and they also can give challenge to top teams and try to catch their chance for the Champions League spot.
Hopefully management will do some positive work for the improvement of the quality because this is surely important task for them if they want to have chance for the top spot which is surely not possible without good players which surely need good funds as well.
Investing more in this transfer for Milan is not as easy as one might think, especially with their current financial problems, of course, this is making it even more difficult for them.
I think instead of making this more investment, it's not even impossible, they will reduce their players, especially now that Madrid's interest in Leao is still there and to maintain their finances, they don't rule out the possibility that they will sell their expensive players.