If you are still having fun with the level 80 content and want to get as much for your money as you can then sure it's viable. If you want to play the same game everyone else is playing then no.

Honestly if you are just doing pve by yourself you could probably just stay with vanilla wow and have fun for many months. If you do everything at level without overgearing learning all those fights with different group compositions would probably be challenging and fun. Of course the cost of the expansion isn't really the main cost with a sub based game so I would definitely lean toward upgrading unless you just can't justify the expense.

As far as entertainment costs go WoW is super cheap. The 2 bills it cost for a 5 boxer to get an expansion(once every couple years) works out to about what it costs for a decent date night. I'm not advocating skipping the date night either, just putting it into perspective