Broswer vendors are in a good position to make this call because they can use telemetry to measure the effectiveness of shared caching. Personally I doubt shared caching is as effective as it used to be. Surely that remaining effectiveness would be decimated by any attempt to implement an isolated-by-default policy that would require website authors to opt in. So all in all disabling the shared cache strikes me as a reasonable option.

Broswser vendors could choose to bundle some popular fonts and libraries but that comes with its' own set of problems.