Law firms are relatively static sized, sign leases of epic duration, and can thus remodel. Startups need to move once a year because they're growing and they keep filling up their space. When you move once a year you're really looking for a sublet, probably already furnished, and choosing from what's on the market.
So at the core, the cause of open floor plans is too many open floor plans. And the more people that switch their space to offices, the more spaces will be available on the market for 1 year sublets. But that's not what's available now, anywhere.
I totally get that there are non-cost reasons, like being able to rapidly expand. I was addressing just the "cost per person" angle. (I basically worked in that scenario before. We started with our own offices, then as we grew we reconfigured to space into large rooms with 4-6 cubes. It worked okay, especially because starting with individual offices enforced a "quiet" culture.)
So at the core, the cause of open floor plans is too many open floor plans. And the more people that switch their space to offices, the more spaces will be available on the market for 1 year sublets. But that's not what's available now, anywhere.