Basically the equivalent of RAID 5 in terms of redundancy.
You don’t even need to do RAIDz expansion, although that feature could save some space. You can just add another redundant set of disks to the existing one. E.g. have a 5-disk RAIDz1 which gives you the space of 4 disks. Then maybe slap on a 2-disk mirror which gives you the space of 1 additional disk. Or another RAIDz1 with however many disks you like. Or a RAIDz2, etc. As long as the newly added space has adequate redundancy of its own, it can be seamlessly added to the existing one, “magically” increasing the available storage space. No fuss.
They can’t. Competition for capital is forcing them to extract the everliving profit out of people. Their competitors would not be far behind on this train if it increases profitability.