Maybe, but it has an increasing contemporary use so to remove / replace it would be a retrograde step:
Increasingly tax authorities are requiring the reporting of tax transactions via Business Identifiers so to replace the existing default field with a custom field would mean that Manager would lose that centralised collection of data to enable the applicable reporting.
Where countries have the requirement that Business Identifier’s are a must on documentation, then under custom fields you would be forcing every Manager user of those countries into custom theme creation.
So if a country has a 1000 users, then you potentially have 1000 custom themes and if Manager in some future update conflicts with those custom themes then you have one holy mess.
Custom Fields are ideal for voluntary data, but the Business Identifier is universally used as legal data and should be managed by the programme and not the playground of the user.