Milan are playing two matches per week for 3 months in a row. If they eliminate Arsenal they'll keep playing two matches per week.
You can see how easily Milan players get injured lately (which wasn't the case last year and the previous years). That's not a coincidence. Older players can not handlle fatigue the way youngsters can. This year i hear Ancelotti, Oddo and Galliani complaining how Milan players are dead tired. Kaka is getting injured way too often. Seedorf plays 1 match and he's injured for the next 3 matches. Nesta is now injured, Gattuso was injured. Not to mention Ronaldo. Pato is getting beaten by the defenders all the time. They have huge problems without squad depth.
If Milan pass Arsenal, i can easily imagine Ancelotti fielding the reserve players in serie A and this year, whenever the reserves played (some 7 or 8 matches) Milan didn't win a match.
It's obvious that Milan can't fight on two fronts. Unlike last year where they had Lazio and Fiorentina with -15 points as their rivals for the 4th place and they managed to get it, this year they have us (no matter what some will say, we are much better than last year's Lazio) and much more solid Fiorentina to beat in order to get the 4th place.
In CL they'll have Europe's best against them.
When i put it this way it looks that it's not so bad after all if Milan eliminates Arsenal

There is a problem in this also..... if Milan win CL and we win the 4th place while Milan finish 5th, then according to UEFA rules Milan will play CL next year while we'll play UEFA Cup.
Now be smart and tell me what would be the best thing to happen. Shall i cheer for Arsenal or for Milan ?