This isn't an argument it was a person asking for folks opinion.A person can hunt with what ever they want with provided it's legal.Speak to an outfitter in area's where the biggest deer reside and ask them for their recommendation versus an Outfitter where the deer are smaller.
On a Canadian mature buck you are giving up a lot of possible kill shot's using a .243.If the deer are smaller where this gentleman hunts than perhaps a .243 will work for him.Personally I would prefer more versatility in a rifle.I hunt big bodied deer every year with a .270 and I do it a deep driving bullet.I have never had an issue.With the bullet variations available in a 30/06 a hunter can cover a pretty big spectrum from light loads or managed recoil loads up to 220 grain bullets.
To each his own!