It's a good job these people don't work in construction or mechanical engineering. They wouldn't survive and would be out of their jobs in a few jobs.
Yes, because predicting the state of an economy in a decades times is exactly the same as laying bricks....
These forecasts should always be taken with a pinch of salt as external factors change everything, they are, however, a group of respected independent academics that very carefully make these forecasts with a lot of evidence to back them up. Doesn't make them right, but they're worth listening to.