Right now, rather than being at 100/0, I'm actually at something like 100/50.
Hiring a staff scientist will send me towards 100/0. Which means not only do I have to think "Can I afford you?" but "Do I want to afford you?"
The amount of time you have to spend writing grants is a function of: 1) how much money you are burning, 2) how much help do you have writing, and 3) how good is your science (better science will be more successful).
No doubt I hurt on #1. But I think I help enough on #2 and #3 to compensate, and therefore I don't think my PI spends a net larger percentage of time writing with me than he would without me. Anecdotally, over my time with him, I think has been a slow decrease in the fraction of his time he spends writing.
I think there's a space where yes, your impact on #1 is probably offset by gains in #2 and #3. But there's also a space, where I'm at right now (and I suspect many new investigators are) where your impact on #1 either moves a staff scientist outside the realm of possibility, or simply dramatically increases the likelihood of stochastic funding failure unless you're literally god's gift to the research world.