Come on guys. A Yes or no will do.
Yes.
Download an option file that includes the EPL and NPower Championship (there's several on this site alone). Then begin a Master League, turning off transfers and player development if you so wish. Voila, you have a single player "League Mode", with actually a little bit more depth since you would have two divisions, as well as the usual FA Cup and Champions League competitions.
So, can a "League Mode" be made? Yes, via the ML options available. Easy enough.
It's threads like these that solidify just how improved PES is now - when the most complaints surround something slightly more trivial, such as a League Mode, as opposed to the gameplay itself, which might I add, is probably the best PES has created in many years. It's probably the best gameplay since PES6 on the Xbox 360 to be honest, and given that sports games are usually repetitive, their strongest asset has to be solid gameplay, in order to have people coming back to continuously play it. PES 2013 has succeeded I firmly believe.