A read-only mode in OJS would be interesting.
Some use cases:
1) Server migration
OJS needs to be offline to extract a dump from the database, copy files, etc. With a read-only mode, it could always be in the air, returning to its full functionality at the end of the migration.
Large OJS installs would benefit most from this feature.
2) OJS upgrades
The upgrade process is often time-consuming, and an OJS read-only mode would allow the public to continue viewing your journals.
3) Journal disabled
There is no more editorial flow and the writing mode just makes it less safe to keep the site up.
Perhaps it is interesting to think with this feature request: