Lu, thanks for your reply. Actually, I agree with you completely on this issue.
SASless, I only wish the world was as black and white as you see it. The truth is, it's complicated and you know it. There obviously could be scenarios in which the things you say make perfect sense. And in those cases, you're absolutely right. If an old aircraft does the job acceptably, then there's no reason to get rid of it, other than the need to have the latest gizmos. However...
Aircraft lease costs are usually deductible. But let's not even get into the tax issues of operating aircraft.
Many pilots are under the (false) assumption that old aircraft have been "paid for many times" and therefore generate nothing but "profit" above their DOC's. In truth, such aircraft are often collateralized for loans, incurring continuing debt service, even if it's not in the way we conventionally think about individual aircraft or auto loans.
When an old aircraft type is phased out for a new one, if the old one can be sold it generates revenue. No matter how the airframe is disposed of, any surplus spares are sold, further offsetting the cost of the new acquisition. If the old type is scrapped, there are even more components to part-out. Does it always come out equally? Of course not, but sometimes there are other financial "incentives" to buying new ones.
How long can airframes fly? I agree with you, that with proper inspections and maintenance, an aircraft should be able to fly indefinitely. But that's a pretty big "if," and unless the manufacturers put a life limit on the airframe itself, then what constitutes an "overhaul?" Would you comfortably buy/fly a 30,000 hour airframe even if it came from a reputable operator? What would it be worth, even with zero-time components?
Helicopters are not aeroplanes. As you yanks are unfortunately seeing with your firefighting aircraft, fatigue takes its toll. (So does madness, but this isn't the Rocky Horror Picture Show, and I'm not Tim Curry.)
Also, airframes get heavier with age due to the patches and reinforcements that come with years of use and abuse. Older airframes will simply require more and more attention to detect the inevitable cracks. Older airframes suffer from more down-time than newer ones. And although it's not directly quantifiable, public, customer and pilot confidence erodes when the airframe passes a certain arbitrary age. The spectre of a catastrophic failure is something none of us likes to think about.
Ultimately and obviously, each operator has to decide which particular aircraft is suitable. Sadly, it's not up to you pilots to dictate anything. However "fiscally irresponsible" things may seem, rest assured that the owners are putting pounds in their pockets. (They just don't want to put them in yours.)