IMHO it was all downhill once VLAN use became the default and viewed as equally secure as physically disparate networks.

We used to physically isolate security domains across the board. Everything is virtualized now, which makes it a whole lot less visible when boundaries are being violated, where it used to be obvious.

From the looks of the Twitter thread pointed out by @ 0xDEFC0DE, looks like the ransonmware was spread by Active Directory group policies, probably by organized crime, as opposed to a random drive-by download infiltrating an entire organization.

Network segregation is a speed bump. See eg Stuxnet.