Creating an active page within a channel
I would create a single template that is displayed when you visit /product and set entry to be the first entry in the Product channel that is live.

{% set entry = craft.entries.section('products').status('live').first() %}

All you need to do then is disabled all product entries apart from the product you want to display.



UPDATE: To avoid duplicate content or different content on the /product page you could redirect to the current product entry page /products/product-a using

{% redirect entry.url %}