I'm not sure I agree it's particularly good leverage since what happens is Apple ends up buying up all of TSMC's production capacity pushing out other customers

I'm not sure I agree it's particularly good leverage since what happens is Apple ends up buying up all of TSMC's production capacity pushing out other customers. There is guaranteed business either way, just Apple is more of a guarantee since they are existing. The other bit is how much Apple invests in TSMC's R&D, which could be a huge loss if Apple moves. But all this is likely just negotiations and Apple won't be making a move (where to anyway?). Maybe instead of 6% per chip Apple invests a bit more elsewhere in the company to keep production costs as low as possible.