I disagree. A general with the right traits will still be better in some cases.
Example: 12 armored divisions led by a general with fortress buster and armor traits attacking a fortress. The army will get the skill bonus+double trait bonus in this particular case.
Looks like FMs will be needed for the masses and generals needed for the small special stuff.
Promoting everyone to FM also sounds like a bad idea because all your leaders will start at one skill level less.
Obviously you will still be able to have Generals along with Field Marshals but it will make sense to make all the Field Marshals day one.
What there probably needs to be is a limit of one FM for every 10 Generals or something like that, but then people will just assign one general per division so they can still have heaps of FM's at the start for the free Poland XP. As far as starting with lower XP I don't think it makes any difference. As Germany for instance you are going to walk over Poland no matter what and get upgrades on the way.