As an interrim solution, and just because I find it more straightforward anyway, I find I tend to wait until I build up the funds to fully purchase/replace a line’s fleet in one go, rather than adding extra vehicles over time. Makes life a hell of a lot easier. Pause, click the send-back-and-sell button, buy a fleet of replacements, assign to line, unpause. Job done.
It’s not an absolute that you have to replace a vehicle the moment its official lifetime is up. I’ve noticed the official lifetime is only really a recommendation… all vehicles will increase their running costs steadily over time, and will continue to after their official sell-by-date. A few years one way or the other isn’t that big a deal. In fact I’ve had horse-carts hang around for over 40 years on occasion, and still remain just about profitable even with +125% running costs 😛
Admittedly, I haven’t actually gotten that far through the game yet, and I’m playing on Hard, so I only have a couple of towns with tram and truck services, and a single train line running between three towns… ~30 trams, ~50 trucks, and 4 trains. I can imagine even with the convenience of quick mass-replacing, it could get unmanagable later in the game when you get up into the multiple-hundreds!