Different workloads require different infrastructure.
Can your workload saturate the TPU without getting throttled by memory or network? Great! Use TPUs and reduce training cost.
But if your TPUs are idle 70% of the time because the constraint is getting data to them ...
"A3 represents the first production-level deployment of its GPU-to-GPU data interface, which allows for sharing data at 200 Gbps while bypassing the host CPU. This interface, which Google calls the Infrastructure Processing Unit (IPU), results in a 10x uplift in available network bandwidth for A3 virtual machines (VM) compared to A2 VMs."