It seems pretty reasonable to me and is super fascinating! In a way it helps the world feel more lived in, the implication being that the stories written are only a mere slice of life to a bigger picture. It leaves plenty of room for fan interpretation and OCs too! There are so many unexplored branch lines like Peel Godred, Kirk Ronan, and Harwick.
It’s also likely that many of these locomotives are for redundancy’s sake. Not all of them necessarily have to be running at a time. This is a revenue earning railway that needs to be able to operate at all times.
Some may be preserved and not actively used for standard traffic. Some examples would likely include Neil, Bloomer, and Aubrey. There are also many other locomotives not even part of this 80 that still inhabit the standard gauge rails!
Bill, Ben, and Mavis are all privately owned, and there may be more like them. We also know that plenty of mainline diesels visit to assist with container traffic.
Sodor is a very lively place indeed!