It's possible to be economically feasible. It requires the ability to pay off the capex. Not only do they have to pay off their loans in record time (prob 2-3 years), they also have to keep spending every 3 years because the failure rate of GPUs is something like 20%. In addition they need to complete building out the stuff they've started, which means not failing to acquire land, energy, and water (the most dangerously rare and absolutely necessary resource for AI), not dealing with collective bargaining, nor any increased shortages or price hikes in materials. So there is a lot of risk.
Ballpark that they need to make around 110 billion a year, each, to break even on these investments. Let's estimate 550 billion a year in necessary profit required for the major frontier companies. That means there needs to be 550 billion of money, available to customers today, that isn't being spent on anything else, that they will now spend on AI. Maybe some of that comes from increased value, efficiency, or layoffs. But 550 billion is not a small amount of money.
Spread over the whole globe, the cash is there. But it is a hunt for cash, combined with a battle to successfully complete their buildouts, keep them running, and make bank, before the bookie comes knocking.
The railroad panics of the 19th century (and subsequent depressions) happened because they over-leveraged private capital without the ability to profit from it quick enough. So it really is a question of 1) can they really build it all, and 2) will people really pay for it all. If either answer is No, we are looking at economic catastrophe.