There is a world of difference between any composite (such as a polymer) and any metallic (such as aluminum).
Fundamentally, metals eventually crack. If the structure and the metal material(s) are well designed and appropriate for the loads, crack initiation and propagation may take millions of cycles; however, eventually all metals crack.
Conversely, composites flex and, therefore, theoretically never crack within design loads. They have other problems -– delamination, debonding, strength/durability on composite/metallic interfaces -– but they should not crack.
Okay, that’s the theory. In practice, a properly designed/maintained handgun firing appropriate pressure loads should last a LONG time. Composites may have a hypothetical advantage compared to metals, but there are lots of 1911A1s and S&W Model 10s out there that have 100,000+ rounds fired with ZERO adverse longevity indications.