Then after 2 years, sell the $1,800 MacBook Air for ~$1,000, the iPad (now presumably 2-3 generations out of date) for ~$300, and the Mac becomes better value.

Then after 2 years, sell the $1,800 MacBook Air for ~$1,000, the iPad (now presumably 2-3 generations out of date) for ~$300, and the Mac becomes better value. You've taken a $800 depreciation hit on the Mac, and although the depreciation on the iPad was only ~$400, you've also spent $480 on your VPS...