Planned obsolescence would be if they made their bricks break on purpose so you had to buy replacement bricks.

Scarcity marketing? I don't know what the exact term would be that you're referring to. But with retirement.

I think it has to do more with Lego not being able to produce thousands of thousands of different sets.

Like you don't just simply ask the Lego printer to make Margaret Hamilton Lego sets for a dozen people then tell it to go back to printing the newest set.

Lego couldn't make every set available if it wanted to