What is the future for an average NAS?

I am considering an NAS for my home network, probably the DLink DNS-323.

Thinking about it, I was wondering what happens in the future. I can envision several different scenarios...
1. It fills up, and I want to upgrade it with larger drives.
2. The drives get old and I decide I want to put in new drives.
3. One of the drives fails, and I want to install a new drive and go on.
4. One of the drives fails, and I don't want to pair one new drive with an old drive, so I want to replace both drives.
5. I buy a whole new NAS to replace the old one.

Scenario 5, you connect both devices and copy from one to the other.
Scenarios 1,2,4, You have to take out the old drives, put in new drives, but there is no way to read the old drives and transfer data to the new drives.
Scenario 3, I don't think I would trust putting my remaining good drive with important data into the machine along with a new drive and trusting the machine to rebuild the array without endangering the data on the only good drive left.

So it seems to me that at the end of the device's life, either when the user thinks it is getting old, or part of it fails, you will need duplicate storage space somewhere to copy all of your data to. This could be a whole new NAS, a USB drive, or a hard drive in a PC. Copy the data to the temporary drive, build the new NAS, copy the data back.

I've never faced this before because I've always upgraded by putting new drives in a PC or going from one PC to another. I've always been able to keep the old drives running and copy straight from them to the new location. This would not be possible using an NAS unless you bought a whole new NAS to copy data to.

Since I'm new to the NAS world, did I overlook something obvious?