How to achieve shorter taxiing time by elongating the airfield
The cost is that it would required a runway twice as long.

Airports already take up enormous amounts of land. Designers try to build airports near major centers, so taking up twice as much valuable land for longer runways near a major population center is a huge cost.

The longer runways also need to be cleaned, maintained, and snow-plowed.

That is a lot of cost and difficulty just to reduce taxi-time by a couple minutes.   After one has taken a 2 hour cross-country flight, 5 minutes of taxi-time is not a point of concern.