This option was introduced for flexibility reasons, but after thinking a little more, it's a bit of a footgun to have a config that won't let you shrink the block cache below a certain size (since that limits what mongotune can do about memory pressure).

There are legitimate reasons for this option to exist, for example if the block cache is identified as being vital to a workload's performance. But on balance, we'd be better off removing the option and seeing if anyone complains.

Note that some DSI configs may need to be altered if this option is removed.