NintailsThere is of course some negatives to implementing league fees (sandbagging first comes to mind), but I think that if league fees are used positively they could improve the scene in EU and create a better comp TF2 experience.
It depends how the money is distributed, they would have to make it so getting even last place in Prem would make you more money than winning Div1 would, or be a lot stricter on what divs people can play in and not allow rule bending.
The biggest negative to adding league fees realistically is the potential player count loss, as ETF2Ls team count is already declining, i'm not sure if they can afford another hit like that, in any case the low divs should be kept free. But honestly, the concept of paying $15 every 2-3 Months for a hobby that most people spend thousands of hours on is not out of this world.
the problem is just how accepting will people in EU be to that idea. NA never had this problem since they have had to deal with league fees since the inception of their league and scene and it never really got questioned.