How do you define "lifetime"? I bought an Assassin in March, and it is undoubtedly a finely crafted piece of bbq equipment, and with proper care, I do expect it to last many years. But it won't last a lifetime . . . certainly not without the periodic significant "turnaround" where all the "perishible" parts get refurbished with new. Anytime you're working with equipment exposed to fire and hot coals for many hours, they will require refurbing from time to time.
So perhaps with solid protection from the elements, and good cleaning and attention to maintenance, 12 to 18 years is reasonable to expect. But in this time, you may have to replace firebox pan, fire grate, gaskets, perhaps even a burned out fuel chute, fuel chute lid, etc. Some repainting in spots will be expected as well.
Now a traditional offset smoker like a Langs built with thick walled steel plate and or pipe, could probably be built to last a lifetime, even considering wall thickness losses due to corrosion, rust, etc. from exposure to rain, snow and ice, perhaps exacerbated by repeated frequent firing of the firebox area.
These are my thoughts on it anyway. Others here probably have some ideas on this as well. If you're talking about a competition smoker, where it will be fired 6 - 8 times a month during the season, (driven hard and put up wet), I'm not sure you'll get as many years out of a Stumps or similar style smoker. YMMV however . . ..