The 243win and 243AI were my favorite cartridge for almost 20 years, but I spent all of that time spending money on custom reamers, dies, and barrels to make it behave like a 6 creed does out of the box. When the 6.5 creed came around, “mildcatting” the 6 creed became too easy - smiths all over had reamers on hand, drop in barrels became readily available, dies ready on the shelf (6.5 creed Redding dies with .267-269” bushings), brass readily available without fireforming - all upside.
Most folks who are happy in their application with a 243win won’t be blown away by converting to a 6 creed. But there are specific aspects of the 6 creed which improve upon the 243win which make the upgrade sensible for a lot of us.