Sure, but not nearly in as large of quantities.

If the value of saudi arabian oil plummets to 1/4th its value, then they are still screwed, even though "oil is still used as raw materials" and it has non-zero value.

Owning something that is almost worthless isn't really much better than owning something that is literally worthless.

First, most plastics are now made from natural gas, something KSA doesn’t produce a huge amount of. They’ve got decent reserves, but their production lags. Qatar produced 70% more LNG last year than KSA. LNG is also kind of a pain to transit by sea, which advantages countries that can produce natural gas closer to the factories (e.g. The United States and Russia, among others).

And second, plastic production consumes a relatively small part of the world petroleum production. Here in the US it’s estimated that about 2.7% of our yearly petroleum consumption. World wide only 4% of world petroleum went into plastics (2012 figure).

So even if plastics continue to be made of oil, plastic consumption remains constant, and there is no major shift in who produces the feedstocks for plastic (all wildly optimistic assumptions), then KSA can expect to keep about 2.7-4% of their current oil export budgets. That is catastrophic for a country like KSA, who needs all the oil money they can get.