Google's John Mueller confirmed that even if you do not have reviews on your product landing pages, you can still use product structured markup/schema on your product pages and gain product rich results in the Google search results page.
You either need reviews, aggregateRating, or offers and if you have that product on sale now, you can also use the offers type. So if you have one or more of these available, you can use product structured data and still be eligible to show product rich results on the page.
John confirmed this on Twitter but it is also in the official docs.
Here is the tweet where he confirmed it:
You need either review, aggregateRating, or offers. If you have the product for sale (an "offer") then that works.
— 🧀 John 🧀 (@JohnMu) September 17, 2021
Forum discussion at Twitter.
Note: This was pre-written and scheduled to be posted today, I am currently offline for Succos.
