did the airport go bankrupt?
Mirabel is very far away from downtown Montreal. There were other factors as well which you can read on wikipedia. However Mirabel is still used today for cargo flights.
Airlines would have preferred to have never started using Mirabel in the first place, however law required that all international flights (except to the USA) use Mirabel. Eventually it was realized that Montreal did not need two major airports and sufficient facilities were built at Dorval and all scheduled service was allowed to move over. Charter service continued at Mirabel for a few more years until that moved over as well.
When Mirabel was built, it was thought that the Montreal area would continue to grow rapidly and that demand would exceed what could be built at Dorval. Additionally, it was planned that a rail link would connect Mirabel with the city. However, Montreal’s growth stalled in the late 70’s and the rail link was never built, making Mirabel a white elephant.
This was common of a lot of public works projects in Quebec in the 60’s and 70’s.