Restore from .db folder for Analysis Services There is an objective to recover cube from long time ago retired MSAS instance. Issue I'm currently standing with is that actual .abf backups are now in a cold storage and recovering them would take a while; however, i do have filesystem backup of Data (as well as logs) folder of filesystem with .db folder (seemingly containing all .dim, .ds and .dsv files).

How feasible it is to attempt to recover cube from said folder rather then proceed with searching for .abf files?
Yes.  Attaching the database files for SSAS is a documented and supported operation.  But unlike SQL Server Database Engine I wouldn't count on it working if the installed version of Analysis Services doesn't match the version that was used to run the database initially.