What are you trying to achieve?

Make it possible to map logs that lack severityText and severityValue values (which is, of course, not the wanted position) to have a more usable initial default mapping until some proper log processing/re-formatting is in place.
The OpenTelemetry spec hints at the usage of INFO level in these situations (see https://opentelemetry.io/docs/specs/otel/logs/data-model/#mapping-of-severitynumber) . From a usability perspective, associating an INFO level to a log with a message indicating a critical error is not the best option and may lead to wrong conclusions.

What did you expect to see?

The OpenTelemetry spec should include an "UNKNOWN" range associated to severityValue 0. This could be used to apply a more usable raw default mapping for logs missing a severity level.