While the new rig will eat a little into current profits it won't deplete them. Then after your current hashing power has paid for the new rig you'll be hashing more power and getting a quicker return, replenishing those lost profits. Several ways to look at it I guess. Didn't want to start a debate, but just how other miners mentally figure out if it's worth to buy a new rig or not. I'm pretty sure there are lots of miners here that have bought gear knowing it would never ROI, but with the help of their current hashing power overlook that and move forward.
There aren't really several ways to look at it at all. It's simple math, there really isn't room for opinion in the matter. If you purchase a device that mines less than it cost to purchase, you will end up with less money then if you didn't buy it at all. This should be intuitively obvious, but if not, feel free to draw up a hypothetical scenario and you will see that there is no situation where buying a machine that costs more than it mines ever results in you having more money than if you don't buy such a machine.
Now obviously many people do this as a hobby or to support the network or for a bunch of reasons other than making money and will buy a machine knowing it will cost them, because they want to. That is certainly opinion.
