I'm not sure an annual charge would necessarily fix the situation for heavy users. It's very similar to the situation of cable and phone companies losing money on heavy users of their service.
The problem cable and phone companies have is that lots of people sign up for unlimited plans when available because of the mental transaction costs of doing anything else. That makes adjusting terms to manage heavy users harder because those adjustments also affect a large slice of customers where the unlimited deal is working as designed.
Just hop on the next train: BahnCard 100 offers 12 months travel on all DB services. It only costs 3,990 EUR for standard class and 6,690 EUR for 1st class.
They also have 25% and 50% discount cards available. The 50% is quite a good deal, as I think I paid mine back within the first week of getting it.
http://www.bahn.com/i/view/DEU/en/prices/germany/bahncard.sh...
Airlines used to think this way - the marginal fuel cost for a seat was something like $10 (essentially it only costs you at takeoff). But then airport improvement fees, security fees, faa upgrade fees meant that they were paying out $100s for passengers who were travelling for free.