It is not a case of neglecting the maintenance. Eventually the vehicle gets to the point that it is no longer safe, because of rust. You take the vehicle for its MOT and they give you an advisory for rust. You know that the following year it is for the scrap yard, so any maintenance you do in the next 12 months is not going to be long term. £100 would be better spent towards a newer vehicle. That was the way my Volvo went.
This car was very well maintained, but eventually it became no longer viable. This was the last photo taken before I parted with it.
It is the same with tradesmen's vehicles. Once the cost of repairing them to make them safe reaches a point, there is no point in spending more money on them. Because of rust, old vehicles are fairly cheap to buy used, especially vans and trucks. Only big companies buy new and they move them on after 3 or 4 years.
There are no grants to convert to CNG. Conversion costs for new vehicles average £1,200 - £1,500 for a 4 cylinder engine, and rise to £2,000+ for V8s, V10s and V12s. Not too bad if you are converting a brand new vehicle (done on the finance package), but prohibitive for a 10+ year old motor.