You do not need a 1024-bit bus to put 128GB of some DDR variant on a GPU. You could do a 512-bit bus with dual rank memory. The 3090 had a 384-bit bus with dual rank memory and going to 512-bit from that is not much of a leap.
This assumes you use 32Gbit chips, which will likely be available in the near future. Interestingly, the GDDR7 specification allows for 64Gbit chips:
> the GDDR7 standard officially adds support for 64Gbit DRAM devices, twice the 32Gbit max capacity of GDDR6/GDDR6X
Yeah, the idea that you're limited by bus width is kind of silly. If you're using ordinary DDR5 then consider that desktops can handle 192GB of memory with a 128-bit memory bus, implying that you get 576GB with a 384-bit bus and 768GB at 512-bit. That's before you even consider using registered memory, which is "more expensive" but not that much more expensive.
And if you want to have some real fun, cause "registered GDDR" to be a thing.
This assumes you use 32Gbit chips, which will likely be available in the near future. Interestingly, the GDDR7 specification allows for 64Gbit chips:
> the GDDR7 standard officially adds support for 64Gbit DRAM devices, twice the 32Gbit max capacity of GDDR6/GDDR6X
https://www.anandtech.com/show/21287/jedec-publishes-gddr7-s...