plastic can be pretty strong, just not yours.
Here's the plastic Ofmega Sintesi pedals on my commuter bike, where they've been used for over 25 years without failure:
I think modern plastic pedals such as the one that failed for the O.P. are not intended to last much longer than the test ride. The shop is supposed to encourage the customer to buy a sturdier pedal, often one of the clipless varieties. There's no way to predict which of the several mutually incompatible clipless systems a customer may already have shoes/cleats for, so a disposable plastic pedal allows this decision to be made at the retail level.