I think all operators will need to have some bores to maintain...

I think all operators will need to have some bores to maintain grades.
I suppose if the trenches are deeper then the requirement for bore supplementation is less.
Regarding your numbers, I assume that the 1/3 of 400ktpa is some sort of guess.
Clearly without bores the brine becomes progressively dilute over time. Any water table that is drawn down over time and then topped up with surface water will become diluted.
A deep trench operation will have a longer life than a shallow trench operation but without drawing brine from deeper levels both scenarios have a limited life but I'm no expert in this stuff.