Having a full set of spares requires zero re-engineering of the existing grid, and will turn a multi-year grid failure catastrophe into a few days to restore critical components and some weeks of restoring full function.
Since our solid solar observation capabilities now give several days notice before arrival of such storms, some could even be pre-positioned at critical substations.
And you are absolutely correct that the DOD would have very good reason to do this - losing the grid nationwide would be an insane level national security risk.
Also, this would mean proactively shutting down the entire grid for days, at the first warning, then requiring days to restore power. So one warning would require a full week of zero power for the continent, and (I expect) saving only part of the grid.
But yeah, putting in some planning to do that would be a smart idea.
In my opinion, the reserve is cheap at 3x the price. $500mil is 7 F35 raptors, and we are buying ~130/year at this point. There are plenty of defense spending justifications for establishing the reserve - it would protect against EMPs and small targeted attacks. We are already seeing small arms attacks on high energy transformers, and last year someone tried to use a drone to take out a substation.
Not surprisingly, the biggest hurdles identified in the report were the needed land acquisitions and the logistics of transporting the large transformers.
https://energy.gov/sites/prod/files/2017/04/f34/Strategic%20...
https://www.everycrsreport.com/reports/R43604.html
https://www.thedrive.com/the-war-zone/43015/likely-drone-att...