BART is sitting on some prime Bay Area route alignment real estate. If it had been built to standard gauge that would have facilitated multi-use, especially of some its under-utilized exurban lines.

I could foresee BART being absorbed into a larger umbrella agency(say State)and extensively reconfigured. Certain lines could be given over entirely to other operations, like the CHSRA.

Standardization offers many benefits, some pretty subtle but cumulative. Oddball gauges usually found their inspiration in some anti-competitive, monopolistic, proprietary scheme. The Erie RR. thought 6' gauge was superior technically but soon found out the loss of compatibility too big a price.