According to the conflict resolution process, that the account with most edits is selected as a primary account for that username, this may sound reasonable for the username that is owned by the same person on all wikimedia sites.
But the problem will come when the same username on those wikimedia sites is owned by different person and they are actively in used. The active account that has registered first (seniority rule) should rather be considered the primary account. Since, I think the person who register first should own that username on the unified wikimedia sites.
Imagine, what if the wikimedia sites have been unified ever since the sites are first established long time ago (that their accounts have never been separated), the person who register first will own that username on all of the wikimedia sites. The person who come after will be unable to use the registered username, and have to choose their alternate username. This logic should also apply on current wikimedia sites, after it have been unified.