The short answer is that in many cases, the correct thing to do is ignore that message and close it without clicking Increase. It is unfortunate that there is no way to suppress that message or “Don’t Show Again.”
I’ve submitted feedback that the yellow alert message is not useful as written. There are several things wrong with how it's written. For example, the message incorrectly says that increasing the cache size will “prevent standard previews from purging.” That is incorrect because if the cache size is increased, the cache will then continue to grow until it hits the new limit which will cause the previews to start purging anyway. The only thing that increasing the cache limit does is briefly delay the time when the cache inevitably becomes full again, and after the yellow alert message is annoyingly displayed again.
If the point of adding the previews cache limit was to prevent the cache from expanding to fill all remaining free space, then when a user sets a cache limit, maintaining that free space is a much higher priority than keeping previews around and the feature should respect that limit and not encourage the user to increase the limit (since that works against the intention of the feature).
The message should just warn that the cache is close to full, so the oldest previews will start purging; and if the user want to keep more previews longer then they can consider increasing the cache limit.
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