I understand 'only' in this idiomatic set phrase to mean something like 'absolutely' suitable/appropriate.
For some reason, I can't get through to the WR dictionary at the moment.
There might be more subtle meanings which lexicographers would explain better.
What have you found out about 'only'?
13) usu ADV , also ADV (emphasis) You can use only to emphasize how appropriate a certain course of action or type of behaviour is. It's only fair to let her know that you intend to apply...
She appeared to have changed considerably, which was only to be expected.