These centres are not for data storage. Legally, my research on human subjects data cannot leave Canada, but we can store the data locally easily.

"produce almost nothing useful" is a naive take on this situation.

Clearly they're producing something useful, otherwise the paid revenue for these companies wouldn't still be increasing so rapidly despite the price increases they're pushing out. Their revenue may not be near the investment at this point, but Amazon lost money for a decade while it got established too. Even if the big companies all stopped training new models tomorrow and laid off all their researchers, there would still be demand for running those existing models.

Besides, what's the loss to Canada if we build a few massive datacenters and then the companies that are using them go belly up? It's not the government building the datacenters, the companies would take a loss and sell them off to someone else to use at a discount. That's happened before, and sparked entirely new innovation due to a glut of cheap specialized resources becoming available.