Hi Jack,
I think the point is that a fast zoom can replace a slow zoom AND primes.
A slow zoom can only be an addition to fast primes (depending when you shoot in low light as well).
I am constantly debating with myself if I shoud stay with my Leica S primes or add the zoom and reduce the number of primes.
However I often do shoot the primes in the f2.5-f4 range.
One solution might be to have 30-90 and therefore replace 35 and 70 primes with a 45 prime (for indoors and lower light).
However if I compare this to the 2 lens set I often carry (either 30or35 +70) the zoom+45 would be slower, heavier and more expensive.
I would gain the 70-90mm range and most important reduce switching lenses.
A faster 30-90 doesn’t make sense IMO, it would be too big and heavy.