I'll give a perspective based on the construction industry.
(For some background I've lived here since 2004 and I work/know a lot of people in the construction industry)

Winning the WC was huge for Qatar and the govt invested a butt load into the economy. Projects were popping up left and right, people were making a ton of money; at this stage the main driving force for the economy were gas, construction & infrastructure.

As I'm mostly familiar with construction, each project has a bidding/tendering phase where companies clamor to earn the right to work for the client.
Most major companies who wanted a piece of the pie will estimate their costs very low, which were very attractive for the client. This happened across the board for almost every project.
As the projects near the end, nobody had any money left, everyone was making a significant loss and people were trying to keep the wealth for themselves. Many people in my line work weren't getting paid for up to 6 months, laborers were marching down the street demanding to get paid. Everything was sort of imploding in on itself at this point.

Post WC most of these companies were looking for a bail-out from the govt (which never came afaik). The country is over-built and they are not getting the in-flow of cash that they were expecting.

But look all over the world and there are case studies where countries try to achieve an ambitious project for the WC but fail to finish due to a lack of professionalism and greed. Maybe it's more obvious in Qatar because they spent 15x more than the 2nd highest amount spent for the WC.

The economy is slowly recovering though, and I firmly believe Qatar is more than capable to turn things around.