[Business case] It would be very useful to a lookup column on the account table pointing to the "top account" of the parental hierarchy. Such a field would allow use cases like aggregation (for charts for instance) or list opportunities (for instance) based on a property of the top account (like the key account manager, the country ...).

[Customization not an option] We planned on created this column ourselves but it happens account is a special entity and it implements limitation of all lookup fields (even custom ones) preventing pointing to the same record. In any table you can create such a custom (of course without hierarchy/cascading... behavior) field and reference the current record. But in the account table, there is a special plugin (Microsoft.Crm.Common.ObjectModel.AccountService) that mistakes all self-referential lookups fields with a hierarchy loop (Creating this parental association would create a loop in this entity hierarchy)

[Support case not an option] This behavior seemed suspicious to me so I created a support case (TrackingID#2409230050003169) but after much back and forth the support team final word is that this behavior is "by design".