It's more than that even. AI may have plenty of utility. But does the massive capex on GPUs that will all be obsolete in a couple years?

You can still run a train on those old tracks. And it'll be competitive. Sure you could build all new tracks, but that's a lot more expensive and difficult. So they'll need to be a whole lot better to beat the established network.

But GPUs? And with how much tech has changed in the last decade or two and might in the next?

We saw cryptocurrency mining go from CPU to GPU to FPGA to ASICs in just a few years.

We can't yet tell where this fad is going. But there's fair reason to believe that, even if AI has tons of utility, the current economics of it might be problematic.

Yeah you'd think the rapid obsolescence of the hardware would dampen P/E ratios a little. It has to pay for itself quickly.

P/E is, after all, given in the implied unit of "years". (Same as other ratios like debt/GDP).