The IRSs are reasonably accurate for cruise flying though, so if you turn off the GPS inputs before you get to an area of known spoofing you'll generally be OK.
You are right that IRS only means your not really lost without GPS but I think (open to correction) one big problem in these days of Performance Based Nav is that a loss of GPS may mean a loss of ability to guarantee whatever Nav Performance is needed for the block of airspace you are in.