There are currently ~11k clans, I think. There are ~48k possible combinations of one, two and three letter unique clan tags. This sounds big at first.
While it is not required, a lot of clans seem to prefer clan tags of 3 characters or less. Numbers are not that common in the tags - if we only use alphabetical characters, the possible tag space falls to ~18k possible unique tags. Of course, not all of these tags are likely to get used, for example "qqq" or "hzg" or any number of other tags. Given this, it seems that already at least half of the 'usable' tags have been allocated, even adjusting for the current clans with longer tags.
If there isn't some kind of purge/expiry period, clans will have to start picking a tag and then choosing a name. It's not the end of the world, but it's not great, especially when the taken tag in question is an empty clan which was previously full of inactive pubstars.
Anyway, not sure where I'm going with this. PURGE THE CLANS