The restarting of the counter on OpenBSD is not what concerns me. The merging of the first file's last line with the second file's first line is.
Someone could have thought it was a good idea and no one was willing to audit all of UCB's scripts to fix the "bug" even if it had been the result of a lazy implementation because ed(1) on a terminal.
So I wouldn't refer to it as "the design". It's not that the methods are unsound, it's that I don't see any method at all.
As for not wanting to break existing scripts, OpenBSD is not even remotely as concerned about backwards compatibility as Linux is. I don't mean that in a derogatory way. It's a perfectly cromulent choice.