I wouldn't say "decades" but several years (five to ten) if it's well taken care of. I've had a Western Digital HDD on my PC for four years and I haven't so much as experienced an issue. A very reliable brand.
Lol, I've had 2 WD element 1tb drives.
Crashed all beyond repair within 2 years.
Still best brand in terms of money/ storage/ compatibilty.
The amount of usage will greatly affect it's lifespan.
You have to consider that just like a Wii DVD drive, the hdd has spinning parts too.
All that moves is vulnerable to wear and tear, no exceptions.
My current wd elements still alive and kicking ( 1 year old )
Just make sure to defragment it regularly if you are writing a lot to it.
it will reduce wear and tear on the motor and heads thus prolonging it's life span.
I suggest using good brand of DVD for storing your backups and a good hdd drive for acctually playing the games.
Then in case of a crash you can easily re-install your backup games from dvd using Wiibackupmanager.
To your new drive.
Also note that dvd's do not have eternal life also.
roughly they will go "bad" in about 5 to ten years depending on brand and dye-quality.
This is without use whatsoever, just storage.
Keep this in mind when trying to keep your collection as long as possible.
The best option is still getting a retail disc and keep it in a save place.
Retail disc won't go bad untill about 50+ years.