Milan already had a strong team and CL football. The only players I'd take from Milan's mercato would be Ibrahimovic and maybe Cassano with some guarantees. The fact is we can't attract players like Ibrahimovic in our current situation so we have to spend €15m on 'mediocre' players like Krasic, Bonucci and Martinez to develope them into top players, reach CL, and then buy more established players to compliment our longer term plan of developing.
On thing you didn't mention is how Milan spend €30m more than Juve per year on salaries, and that is before Cassano, Emanuelson and Van Bommel, contrast that with Toni who we have added on €1.5m. So there is a difference of over €35m, which for us is two top class players. Ibrahimovic is on €9m a year, completely unthinkable for us.
I agree with you that Milan is most likely paying a significnatly higher amount for salaries but that is the result of failed policies of the last 3-4 years when contracts, RICH contracts, were given to players who were approaching 30+ and whose contribution to the team was grossly overestimated - you have your Grossos and Amauris, Milan have their Jankulovskis and Kaladzes and what not.
Furthermore, a vast portion of the salaries at Milan go to players whose contracts are expiring next year and there is good chance they will be either redimensionalised or simply not renewed - several steps in the right direction unlike previous years.
Anyways, my point was about the new players coming in - they have not cost a lot while their salaries are rarely more than 2mil/season, which is laughable for the standards Milan have set in previous years. Sure Ibra is an exception - those 9 mil include incentives based on what the club wins and how the player performs - but still, a player like that I am sure juve could have more than afforded by cutting their dead weight elsewhere - e.g. when the team needs rebuilding, having a 33-34 yo GC who commands salary of 5-6 mil or so, if I am not mistaken, is pure luxury.
Furthermore, the way the Ibra deal is structured would ease the financial burden from his salary - Ibra is on a free "loan" this 1st season, while Milan has to pay 24 mil spread over the next 3 years (counting from the end of this season) ... I believe that it is exactly this that allowed Milan to afford Ibra as his salary was mentioned many times as the reason why his coming to Milan would be unthinkable in the weeks before the transfer.
As for paying 15 mil for average players, 15 mil is no peanuts, that amount, when wisely spent could very well bring in 1 and even 2 excellent players as opposed to 1 average player - it all comes down to the choices being made - I hardly need to remind you how many players, better than all new Juve reinforcements, were bought by smaller italian clube like Napoli, Lazio, etc for pretty low transfer sums and salaries - obviously, the better options to spend that 50 mil or so Juve did this last transfer season were available ... the problem was that the people who were making the decisions didn't see them.
At the end of the day, there is alsways a way around obstacles, you just have to be smart enough to figure that out - something both Milan and Juve lacked in the years after calciopoli. Lately, Milan seems to have woken up, my guess is that Juve needs to get rid of Marrotta and some others before they do the same.