I have 2 setups...
Main Residence: Drobo with 7Tb hooked to my oldish White iMac
Second Place: Mac Mini with 1 x 2Tb WD Drive, plus a 1Tb 2.5" passport drive.
The Second place setup is a backup of the main residence covering about 80% of the date. It'll be upgraded soon to accommodate my expanded storage in the drobo.
Apple wont actually 'store' a copy of your purchases in its data centres, just a database of what you have bought that allows you to dload it when you want from there central library of files.
However.......the internet just wont support that sort of traffic as a norm from a growing numbers of users. With a couple of TBs of movies on my system, plus nearly equal amount in TV and personal video files, I cant seriously see anyone offering that as a viable option for at least another decade.