I think you must be misunderstanding me. I am clearly not saying you can pin this down exactly. That would be an absurd thing to say. I am saying you can analyze existing statistics to make an educated guess at what a break even number is, and make decisions based on that, and how you think the conditions may change.
This isn't a new concept in crypto. Difficulty for PoW coins is a mathematical best guess based on current hashrates to best approximate a target block time. This isn't exact, and yet, this we still deem difficulty to be an important metric to look at. Profitability calculators will calculate your estimated break even time. This too is inexact, but very useful in trying to plan hardware purchases for mining.
Similarly, it is clearly useful to do some analysis of forging pools, if your purpose is to lease your balance with the hopes of earning more NXT than if you hadn't. So yes, you run the numbers to produce a number which assumes nothing in the Nxt ecosystem changes. That is your benchmark with which you then project your opinions about how things will change. "Sure, the numbers say I am projected to earn less than the break even point of 3 NXT based on this pool's forging history, but I am predicting tons of new assets being added and lots of 1000 fee blocks, so I am going to do it anyway."
As it stands, much of the tone I have noticed after the release of pooled forging has been "Don't just let your NXT sit there, put them to work for you in a forging pool!", with the assumption that doing so is better than not. This assumption is not necessarily correct (clearly, as the vast majority of the people in this pool are in the red).
All I am saying is that an analysis of the current state of these forging pools is a very useful piece of data to have when deciding if it is worth it or not for you to lease your balance. It is not the only piece. It is not exact. Of course not. It is supplemented by your beliefs and predictions of what will happen next. But if you think using "math" to make calculated estimates to inform your decision regarding wether or not this makes sense isn't helpful here, then we probably just won't be on the same page.