Galaxy Store is a garbage adware. Always runs in background. And if you accidentally open it it will start downloading "recommended apps" without your permission.
Otherwise, I completely agree. Samsung should just use Google Play or the System update process to manage system app updates, etc.
I can see their concern with relying completely on gPlay, as there may be instances where the permissions needed for a system app go beyond the allowances of gPlay, but there are other ways to avoid those issues, and the Galaxy Store is trash.