If the comments below the story are correct, Boeing has other airframes to take any future test workloads.

The lack of standardization has sent many prototypes and early production airframes to early retirement.

Basically, if they have other test aircraft, other demonstrators and it can’t be sold for commercial use, then it makes sense to retire it.
Besides, the article makes it clear it will be stored for the time being, it could be reused as an avionics test platform…that’s what happened to the 767 prototype and IIRC, the 757 as well.