I am unsure about the historical reasons for moving from 32-bit to 64-bit, but wouldnt the address space be a significantly larger factor? Like you said, CPUs have had vectoring instructions for a long time, and we wouldn’t move to 128-bit architectures just to be able to compute with numbers of those size. Memory bandwidth is, also as you say, limited by the bus widths and not the processor architecture. IMO, the most important reason that we transitioned to 64-bit is primarily for the larger address address space without having to use stupidly complex memory mapping schemes. There are also some types of numbers like timestamps and counters that profit from 64-bit, but even here I am not sure if the conplex architecture would yield a net slowdown or speedup.
To answer the original question: 128 bits would have no helpful benefit for the address space (already massive) and probably just slow everyday calculations down.