That's the most interesting part to me which will play the major role in how WFH situation will evolve. I 100% understand the logic in such situations, but:

1) If company like Twitter is saying: "From tomorrow our default is work from home, but you all get 20% salary cut". This will be essentially a salary cut, nothing more. If let's say Facebook says WFH=on-site developers from Twitter will be flowing to Facebook because of 20% salary cut

2) In order for everyone in FAANG to say simultaneously that we have to cut salaries 20% simply because it is WFH now - there should be a strong evidence that productivity in remote workers is 20% less and they will need to hire 20% more engineers to have the same amount of work done. If it is not correct then companies have incentive to drive this number down to 10% or 0%

3) FAANG and overall Valley residents must also consider the long term effect on their community. I doubt that Silicon Valley will survive going 100% virtual and concentration of innovation can be lost if people go 100% remote. So they might impose 10-20% cut on a premise that they want to create a community on-site. Not sure how C-suits evaluate such factor.

Overall I suppose if companies go WFH by default they will need to reevaluate their incentives structures to be competitive.