I agree on this, I would pay to fuse my accounts together as I would stop paying for my second account at WoTLK anyway.
However they should implement the following limitations:
The accounts do not need to be on same battle.net account but require an authenticator connected to both. In the case of not being on the same battle.net account a confirmation needs to be send to both account e-mails , this needs to be accepted by both.
Can’t transfer a boosted character if there is already a boosted character on the target account.
The account used to transfer a character will be linked to the target account permenantly (or perhaps for few months) and can only transfer more characters to that same account to discourage buying a character from boosting services.
I believe this would be reasonable.
And now even more so, people playing on 5 accounts taking up slots on already full servers.