Do you think in a society or environment with no male influences, no males to please or titialate, no pressure to look good for men, etc. with 100% free choice what to wear, women would always wear (dare I say it) modest clothing?
Going by Nuns I Have Known, yes and no.
Nuns wear shorts, short sleeves, and don't cover their hair, if they are in an order where dress is not prescribed. Younger nuns dress less modestly than older nuns.
The 30-something primary school teacher nun I used to know wore more or less the same as the other primary school teachers (practical skirt i.e. wide with opaque tights in winter, or practical trousers i.e. flexible, long or short sleeved top depending on the weather).
The 60- and 70-something retired teacher/nurse/admin professional/academic nuns wore more or less the same as other older, middle-class retired professional single women - again trousers that were comfortable but not overly loose or tight, long or short sleeved tops.
None of them made a big thing of covering their bottoms (this is a cultural thing as many "non-modest" groups in some countries regard this as important too - e.g. Christians in a majority Muslim area I used to live in would not usually tuck in their blouses and would go for longer tops) or their hair, nor of wearing loose clothes, which seem to be the hallmarks of "modest" clothing.
But apart from longer shorts in very hot weather, and normal (not burkini or long) swim suits (this was before tankinis/swim shorts were very common), most of them would not wear very short, tight, or revealing clothes (no low cut tops, but not wearing them right up to their chins either).