Well, auto replace worked well in Open TTD. But, that game required routine maintenance of vehicles, so you would have to include a trip to the depot regularly or have the route pass very close to a depot. If the age of the vehicle was up, it would replace itself when it went in for routine maintenance. Personally, I think doing that has better immersion than simply deducting money every month from your vehicles; it still involves the running costs, but in a more realistic way.
Also, routine maintenance prevented the vehicles from breaking down and blocking traffic which added more to the realism; ie: the older and more infrequently the vehicle was maintained, the more frequently it would break down.
And, @gGeorg: Although a video diary would be a nice touch, making a 20 minute video can take up to a week to produce, so it would be quite unpractical and would take them away from actually working on the game itself. The fact that the devs actually appear on these forums shows that they do read these comments, which is more than what other developers do. I think that is an encouraging sign by itself and, from reading their comments, I know that they have already mentioned that they’re focusing on performance at the moment which would arguably be the most important issue for the game right now. The other “nice” features will have to wait for the performance issues to be solved first; all of which will take time.
In the meantime, I’ve got several other games on the go with similar problems. Between them, updates occur quite regularly so I can switch between them as they are released. You’ll go crazy if you only have one game that you’re hanging out for. 😀