I suspect the key to success will be managing the drop off of TS and the build up of TSW. If they get it right then one should go down as the other goes up and they'll be fine. Mess it up and they'll go a year without any income.
I'm going to have much the same problem
However I am currently of the mind that those with a bent for trains more than 15 years old will likely stick with TS longer than others - based purely on the fact that DTG have shown little interest in older railways compared to modern ones.
Don't forget DTG will also get an instant cash influx when TSW goes on sale which will be bound to bring in more than any DLC as everyone who wants to buy future TSW DLC will need to have the core game.
The interesting thing today was reading DTG's announcement http://train-simulator.com/wp-content/u ... raphic.jpg
The text at the bottom referring to having to buy TSW: CSX Heavy Haul when it comes out makes me think that it WILL only be the CSX route and locos that come with the initial release. Which seeing DTG is a UK company is rather disappointing but I guess they need to follow the money overseas.