This is a workaround for T58473 to avoid likely event of ruining sculpted data.
Personally I would force sculpting to always work on the highest level, removing the sculpt level from the interface and making it equal to totlvl.
The warning is better than nothing, but it's still hard to understand for a user what it means exactly or what they are expected to do to solve it.
@Brecht Van Lommel (brecht), thanks for the review!
I agree with you, and main motivation for going the warning level was
the more localized nature of changes. If you're around to give extra
pair of eyes to the force-highest-level-sculpt lets go that root!
On a positive side this approach doesn't require translators to rush
to translate yet another string introduced in the interface.