Because you'd need to trash the old GPUs in order to make room for new GPUs. Right now new GPUs get online mostly in new DCs. TSMC fab capacity is much more limiting than DC building and it will likely keep being the case. It's much easier to build a DC than a fab.

Because they are >10x more power efficient.

If silicon were relatively abundant and power/DC space scarce, you'd get an order of magnitude more bang for the Watt by replacing the H100s with newer GPUs.

But nobody is doing that. Blackwells are being installed as additional capacity, not Hopper replacements.

So it is pretty clear that silicon is the primary bottleneck.