Once molars are intruded after invisalign, can they naturally extrude over time by not wearing the retainers?

Maybe: Sounds like maybe your bite maybe a little off. Normally after most orthodontic treatment there needs to be an accusal (bite) adjustment to correct some of the premature contact areas. Naturally teeth extrude to meet the opposing dentition but You need to see the orthodontist or the treating Doctor to double check your bite. Hope that helps.

Yes: If you do not intend to wear retainers exactly as prescribed, don't waste your time and money on clear aligner treatment. As aligners are not as good as braces in correcting malocclusion, and end result not as fine tuned as with braces, post-treatment stability is less assured. Retainer compliance is mandatory.

Of course: This movement occurs very slowly. However, it can be prevented through continued wear of retainers. Take care.