It seems (from this meta thread) that we are no longer intending to retire sandboxes, and I now approve of this plan. Because the answers can be sorted by activity, old posts will not clutter the sandbox and there is no need to retire it.
If the current sandbox will not be retired, it makes sense not to leave the numbering stuck at XIV forever. The numbers in the title imply that sandboxes are going to be retired eventually, but if this is no longer the case, it is worth changing the title of the Mark XIV sandbox to reflect that it is now The Ultimate And Final Sandbox.
It was also mentioned elsewhere on meta that the numbers confuse new users.