People want to have their cake and eat it too. Silicon Valley companies pay very well because cost of living is very high in the area and they _need_ to pay well to attract talent in that area. If remote work becomes the norm, pay will start to even out across the country. If you live in a place that didn't pay well before, you'll be better off, but if you live in a high cost of work area, you'll probably be worse of…
The demand for good engineers far outstrips the supply, so companies are forced to compete with each other, which is why you get lots of other perks other than salary (free gym, free lunch, free dry cleaning, etc). Any big tech co that decide to pay the same regardless of location will have a huge advantage recruiting and maintaining remote employees. If a remote employee's only option is to jump to another FAANG company for a 15% salary decrease, what motivation will there be to leave?