1. The question is at the same time about why should overlapping `=terminal` inside another one be valid. This makes them redundant, and causes definition and consistency problems. Besides, how do you know why they exist as `=terminal` + `building=` ? Users may not know other solutions, they didn't touch the `building=` , or someone like you added the`terminal` back, etc. There are many possibilities. What I want to show there is as titled, and your point 2, about the possibility of `=terminal` `=multipolygon` without `building=` . 
3. They can still understand the `building=` . What's the difference? When they need more detailed info, they have to look at `building=` ,  `building:part=` , etc anyway. 
4. Skypier is a different tab or bookmark, They are simply shown as next to each other on the map. What I want to ask is if main T1 + NSC is 1 `building=` , how should the `=terminal` be handled? If the examples show `building:part=` + `=terminal` is also used, is `=terminal` really defined as always `=terminal` + `building=` ?  Many `=terminal` are also wrongly duplicated.

There are 5024 `wr[aeroway=terminal][!name];` results , that's more than half of the 9804 instances in total. Many could be valid as the unnamed only one in many small airports, but many more look invalid as being close to each other. It seems  `=terminal` is not mostly used properly.   