Sounds like a pretty good deal.
What you didn't take into you account as well was the fact that this is $3,000 if Ethereum doubles in price, $4,500 if it tripes like you specified.
The only way to avoid diminishing mining returns as difficulty increases is to swap to a new, lesser known coin. Eventually you run out of coins to swap to or the ones you are mining don't go up in value (because if they did, bigger miners would come in and knock you out of the game as the difficulty shoots up). All the time your GPUs are ageing too.
If you want a steady income with arguably a lesser risk profile and more staying power than personal mining offers, why not put some money in to coins that offer node payouts?
As soon as a coin's value increases, more mining hash power enters the scene and difficulty shoots up (ruining your profit margin). You then swap to another coin, eventually the same thing happens. Rinse, repeat.
During all of this, you still have to make back your initial investment on the GPUs and power, then you have usually another 6-9 months max before your entire rig is too outdated and you have to start again buying newer, faster cards (which will then take you another 3-5 months just to earn back your cost).
Just seems a very painful and slow way to cash in on the crypto bonanza (which isn't going to last forever).
There's no way all these coins [1] retain proper value compared to NASDAQ [2]. Take AAPL for example: ~48% [3] in one year. GOOGL for example: ~36% [4] on one year.
[1] https://whattomine.com/calculators
[2] http://money.cnn.com/data/hotstocks/index.html