Putting more fibers in a cable sounds low-tech.
Perhaps the hard part is reassembling the data stream at the far end because each strand’s going to have a slightly different travel time.
In the old days a 9-track tape drive on the mainframes with each track doing 6250 bits/inch had to buffer each track because the tape would shift slightly and corresponding bits from each stream wouldn’t hit the heads exactly the same time.
A friend used to say: “Never underestimate the bandwidth of a station wagon full of 9 track tape”.