Why it looks like planned obsolescence not applicable at PC mobo ?

[golden_labels]

The key word is: intentions.

The concept of planned obsolescence suggests that one of the design goals is to make a product not survive for too long. That this is literally a consciously made decision to limit product life, with the intention of forcing customers to buy a new one. Putting that into an image: the boss comes to the engineers and tells them — nah, nah, this design will work for 10 years. Make it work for no longer than 5 years, so we could double profit.

Design life is a kind of opposite. Knowing that everything must inevitably break at some point, one is forced to choose a design that will oppose the unavoidable fate for at least the specified time. It’s not a step one can skip. It’s not something one does because it gives profit. It’s something that simply must be done for any product, as it’s its inherent propery. The intention here is not artificially shortening item’s life, but delivering the product in the first place.

[SilverSolder]

The key word is: intentions.

The concept of planned obsolescence suggests that one of the design goals is to make a product not survive for too long. That this is literally a consciously made decision to limit product life, with the intention of forcing customers to buy a new one.
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But setting a specific design life does exactly what you say - it makes the product not survive for too long.  It avoids a mechanical engineer putting a heavy cast iron back axle on a passenger car that would last a million miles or more in that application, and instead choose or design components that are overwhelmingly likely to live the design life - and no more. 

To your point on "intention", a car will wear out in accordance with its fully intended 10 year/100K mile design life and the environment in which it is used, including the owner's diligence and care.  Car makers plan their entire production runs and quantities based this fact.  They intend their product to last 10 years, they plan for it, and the whole chain from digging up raw material to supplying spare parts depend on that limitation. 

Car makers don't even compete on this parameter - you cannot buy a normal passenger car that is not designed for 10 year / 100K miles, this specification is treated as holy scriptures by everyone in the industry including component suppliers. 

This design specification is so deeply ingrained that most people aren't even aware that it's there -  instead, they see it as a natural thing that cars wear out when they do, and accept that they have to buy a new one regularly.

Some car makers pretend their vehicles are more solid or long lasting than others (e.g. Mercedes?) but the truth is that unlike classic Mercedes models, the new ones are all built to the same specs as all the other cars...

[...]
Putting that into an image: the boss comes to the engineers and tells them — nah, nah, this design will work for 10 years. Make it work for no longer than 5 years, so we could double profit.

The boss (of a large company) will have thought this through together with sales & marketing and the engineering team.  They will have looked at what is a realistic life from a technology/cost perspective, as well as the implications for replacement rates and profitability - it is all part of an overall mix of considerations.   That's how they end up with 10, 5, or 2 years design life.

[...]
Design life [...] is not artificially shortening item’s life, but delivering the product in the first place.

It is both!  E.g. by deciding to build cars to the 10Y/100K specification, we have artificially shortened the life of the vehicle compared to choosing 20Y/500K as the spec.

A design life specification is what it is:  a limit on the expected life...   it does NOT reflect how long we could make it last if we really wanted to!

[SilverSolder]

Continuing the design life & planned obsolescence discussion a little further...  consider modern cell phones & tablets (iPads).

Late last year, the battery in my wife's iPad wore out after 3 years of enthusiastic use.  It is not possible for the user to change the battery in an iPad, even though it has traditionally always been understood to be a wear item that is not expected to last as long as the equipment it powers.  This irritated my wife; so she took it to the Apple store and told them she wanted a new battery. 

The Apple store reps tried hard to get her to upgrade to a newer model, but she wasn't having it - "why would I pay to upgrade to a model with a slightly smaller screen that does basically the same as the old model" was her stance.   After failing to persuade her of the benefits of upgrading...   they let her pay the $50 fee to change the battery and took her iPad.

A couple of days later, she got a call saying it was fixed.  She showed up at the Apple store... where she promptly received a brand new iPad (of the same model as hers)! 

So, the design life of an iPad appears to be limited by its battery, and Apple clearly had no intention of ever changing a battery.  Here we have an example of a conscious decision to limit the product life to the life of a component that is normally considered a wear component...

[golden_labels]

But setting a specific design life does exactly what you say - it makes the product not survive for too long.

No, it is not — and both my and tooki’s posts explained the difference. Everyone is free to ignore that help and, instead, focus on fishing for similarities in vocabulary describing two concepts or going deeper into fallacies, but I suppose in such a case my job here is done. 

[SilverSolder]

But setting a specific design life does exactly what you say - it makes the product not survive for too long.

No, it is not — and both my and tooki’s posts explained the difference. Everyone is free to ignore that help and, instead, focus on fishing for similarities in vocabulary describing two concepts or going deeper into fallacies, but I suppose in such a case my job here is done. 

We'll have to agree to disagree.