Your well guy is probably right.
A new pump can be a lot of prices. If your going to install it, it's much lower than if a well company does it. For a 40' well a 1/2hp 10 gpm pump is all you need for your home and my price is $344.40 if you install it.
The added homes in your area can't be effecting your water with a 40' well. If they did you would have no water.
I would thing you have a hole in the droppipe, or in the pipe leading to the house, or the pumps impellers are worn out.