even when there was patriotism the legionaires wanted land and other spoils. Hard to show an age of pure patriotism making thousands rush into the army.
Everybody likes to get paid. But then came the age of "bread and circuses". What is the allure of getting some land of your own that you can farm and raise a family on, when there's free stuff given to you for just voting for it?
One big factor in the decline was the wealthy getting slaves, and those slaves working more cheaply than the old Roman middle class. Something like how the Cheap Labor Express operates today.