I'm an old Windows/CCleaner user but have 100% left M$ a couple(?) years ago for Apple with a MacBook Pro 15", touch bar, 4 USB-c ports, and yes--a SSD. (Also an iPhone 7 plus and an Apple Watch too.) As some of you might imagine I have spent many hours with Apple Care. I have one outstanding months long unsolved problem to share elsewhere here maybe. I suspect talking to Apple Care that CCleaner might be a great 3rd party tool that Apple needs however they repeatedly don't recommend CCleaner. And only recently have I gotten some okays to run Malwarebytes--neither of which are found in the App Store. Apple's concern may be that running CCleaner will (and does) mess up some things?
IMO, Piriform's goal should be to get CCleaner in Apple's App Store de-tuning it down to where it causes no problems. ...I'm waiting.
App Sandboxing--fascinating read... So what is left of CCleaner if it goes along with App Sandboxing and could then be listed in the App Store? I wonder if this is the same reason why Malwarebytes isn't listed in the App Store?