When I install a new version of Photoshop Elements, it converts the old catalog but does not remove the old programs. I just installed PS 11 over PS 10 and it did not remove it. I have had the same problem with each upgrade. I have to manually uninstall the old version.
It's good to have the software you paid for
Uninstalling the older version post an upgrade might be discomforting to many users, so...
Catalog conversion works fine irrespective of whether you uninstall or do not uninstall the older version since the catalog folders are not touched during uninstall.
Many people prefer to have the older versions as well. Sometimes there are changes in how things work from one version to another, so someone might want to keep an older version to use a particular function there.