The economic lifespan ends when the cost of electricity is higher than the value of mined bitcoin (I see no reason to mine after that point, except if heat is a a desirable by-product).
Some old ASICs have all ready been turned off for this reason, right?
Agreed in general. It is possible that some miners will continue to operate some old equipment past the immediate economic value, and that may be due to speculation that bitcoins will be worth much more in the future and are worth generating at a loss... but the vast majority of miners will be switching off before that point.