"Don't be silly! Linear A will never be forgotten, it's too simple and too many people know it!"

In one or two cases the knowledge may have been suppressed, but for the most part I see it mostly being a case of a superior item coming along, relegating it to a chemical curiosity for a thousand years or more, and then have the majority of the population go away. In some cases a more guildlike attitude toward knowledge may help.

Finally, I tend to think the major Empires are a little lax on some elements of chemistry in general, since they have other areas to focus on.