Ivan,
This default setting is unintuitive and unexpected, especially if done under the hood. IMHO, since the interrupt handlers are very frequently defined with weak substitutes, the default archiving of the /lib folder resulted in two weeks of frustration with platformio for me. It is not same for other conventional tools.
I placed the /rtos in the /lib folder, and the known working program stopped working. The story is here:
I think the default decision is wrong, and the tool should default to “false” always. If the developer knows what they are doing, they may explicitly alter the default. And it should be spelled in bold in “Beware” section. And also the lib_archive must be always included in the automatically generated (.ini) file, with a comment what it does and what will happen when it is changed.