Ouch.. So Android Q will bring major changes to the way apps can access files.
'Android Q gives each app an isolated storage sandbox into an external storage device, such as /sdcard. No other app can directly access your app's sandboxed files.'
Sample packs anyone? Android Q will be a lot more restrictive in general regarding how apps can access files which is going to be a real headache.
I don't imagine Google would completely block Android Q users from running older apps but whether they would all still be able to function as intended is questionable.
I'm not clear on all the fine details and I don't think there's going to be any issues in the near term but I think there's cause for concern.