First Transit does just that in the US and Canada and is arguably the steadiest performer!
UK Bus was the cash cow returning double digit margins but, as you say, Moir was about being the biggest. As with any bovine, you have to feed it sufficiently or the yield will drop - that's what happened.
We all know the history and can speculate about what may have happened had they sold off Greyhound when they bought Laidlaw, or if they'd done x or y.
The here and now is that it is a tough world for bus operators and especially in areas that were once powerhouses and are now economically deprived. Add in congestion, Uber, cuts to concessionary schemes, and the loss of "bunce" of tendered work on evenings and Sundays and it's a mix that isn't too appealing.