This is a subject near and dear to my heart, as someone who has taken a lot of shit for continuing to run a plastic bottle on a concours car of historical significance

IMHO, there are three factors to consider when running a plastic coolant header bottle:

1) making sure that the temp / pressure of the coolant within the bottle never exceeded the design limitations of the plastic used in construction of the bottle;

2) making sure that the freezing point of the coolant within the bottle was always rated below the experienced ambient temperatures of the environment in which the car finds itself in; and

3) making sure that the bottle wasn't exposed to improper long term outdoor storage (exposure to ozone, UV, IR, and extreme swings in humidity will wear on plastics in a similar fashion to how they wear on rubber components).

If you do these three things (and can verify that they've always been religiously adhered to throughout the life of the bottle/car), I'd personally be extremely confident in running the plastic coolant header bottle indefinitely into the future, and wouldn't have any reservations about doing so.

If you can't accurately verify the historical experiences of the bottle though, then switching to a metal bottle is indeed cheap insurance against a potential future failure.

So, there's my two cents, for whatever they're worth...