They disable bad parts of the die, like bad sectors on a disk.

At that point, is there really a benefit of having it all on one wafer, as opposed to just using chiplets?

Presumably there is, but wonder the same thing.

With wafer scale the amount of connections that can cross a border between two tiles is limited by the spacing rules for the top metal layers and by the number of layers you can dedicate to this (1 or 2, I would guess).

For TSMC N7 Design Rules[0] the pitch is 720nm for the top two metals (1388 wires per mm) and 76nm for the middle metal layers (13157 wires per mm).

In the case of chiplets on an interposer, if we suppose that the interposer can use a process similar to the top layers of the wafer then the number of wires between chiplets is the same. But you need pads and solder balls from the chipets to the interposer (and back on the other side). These pads can be in a grid pattern (like in a BGA package) so the area is the limit instead of perimeter. Let's be optimistic and suppose a pitch of 30µm for the copper pillar micro-bumps.

With a conservative 1000 wires per mm of perimeter the pads are not the limit with a chiplet larger than 3.6mm per side. With a more aggressive 10000 wires per mm the chiplets would have to be larger than 36mm per side, which is just beyond the reticle size limit of many chip fabs.

[0] https://en.wikichip.org/wiki/7_nm_lithography_process

This only works for some things, if you lose a clock or some part of the chip is just spewing garbage into whatever bus it’s using, you’re hosed.

They fuse it off