Pentax plastic bayonet lenses

I have seen my new Pentax K-x kit lenses (18-55 & 55-300) with plastic bayonet each ,that make me surprising especially for 55-300 which is relatively long lens and it was better to make it with steel bayonet.Im not sure about the durability of such lenses even i know that some plastic bayonets are still used for more than 5 years .