Many opens source projects which used Mercurial chose BitBucket to host repositories. Now that Mercurial support has been dropped, many repos which are not actively maintained anymore but nevertheless useful for the open source community will be lost.
But this is also an issue for paying customers, which had Mercurial repos in BitBucket. In many external places there may be links to source code or commits in BitBucket. These will all become invalid.
This situation would be less problematic if Mercurial repos in BitBucket are kept in a frozen / archived / read-only stage.
This would also be a positive sign in general. The decision to completely drop Mercurial from BitBucket does not motivate me to choose Atlassian products in the future, because there is this bad feeling that things might go away with relatively short notice.