I think at least for buses/trucks/trams, an ideal tool would let you set the type of vehicle allocated to the route, and the number of them, plus a “replace after x years old” field. If you want to refit a fleet of horse-and-carts with the latest trucks, you don’t necessarily want the same number, as the new vehicles will be far more effective. A fleet refit could just ask you to update the vehicle type and quantity for the line, and once you hit “okay”, it would handle the selling and buying.
You could even give the refit a priority, so you could order several refits at once, and it’ll buy the highest priority vehicles first as funds become available.
I’m not so sure about trains… there are a lot more complications. I suppose it could still be given as an option, and then it’s up to you whether you want to use it, or handle replacement manually.
I’ve been playing on Hard difficulty, and when it comes to replacing trains, or even refitting with extra carriages, I have to be incredibly careful that I don’t take them out of commision at the wrong moment, or I risk setting myself back a couple of years in lost revenue! Hard mode is brutal 😛