Random content section for SEO, to keepeing it "fresh"

I have an ecommerce site, that has some ranking product-category pages them are hardly ever refreshes. How can I keep it "fresh" and updated in the eyes of google?

There is no new product in the category, and never will be. Just imagine some iphone 8 cases, in a category. There wont be new products appearing in the category, but i have to sell the rest of it.

The problem is, as i understand google devaluates pages, and slows indexing if there is nothing new on it. Then my page goes down on the serp line of course. (been there)
How can I keep it up and fresh? I have tried making clickable related article contents on some pages under to product section, but doesnt seem to have effect, an nonetheless, it will be outdated 2 months from now also...
Tried to point pbns for them but thats not sustainable because, the example "iphone 8" category will be dead weight for the site in a matter of time.

My new idea is to display random reviews at the end of the product category. Every time google indexing the page, it will have relevant "new" reviews. Like if they were real ones appearing over time from costumers...
Has anyone has any experience of this? Or any other idea to keep a page fresh without new content?

I never owned ecommerce sites, so probably that my advice is not the best you can get. But until someone else replies, here are my thoughts...

I don't think that google treats ecom sites like regular sites (blogs / content sites), so you shouldn't worry about adding fresh content to product pages like iphone cases... or to any product page that is. Again, it's the nature of ecom sites (little content) that warrants not having to keep them updated with fresh content, google understands this so I don't think they'll penalize you for lack of fresh content...

And I don't think you should bother with adding reviews, either. Google have made ranking review pages a hell lately, you'd have to do everything google says in order for your reviews to rank. And speaking of ranking, you might actually even de-rank your pages if you add reviews on them because if you don't follow google's "guidelines" (read: demands), you risk losing the rankings of the pages that at least rank somewhere. So, again, it's a bad idea mixing product reviews and ecom sites... at least that's my opinion. If I'm wrong I'll gladly concede if someone knows better...

What you can do instead (assuming that your niche is indeed "iphones") is add a blog to your site and talk about iphone-related stuff (cases, iphone versions, features, apps, etc.) on your blog and then link to relevant products in your store. I think this might give you a chance of ranking for more keywords, but you'd obviously have to find good keywords and create rankable content around them, which can be very hard if the niche is not big enough (there aren't many informational keywords to target). But I think it's still the best option...

As for building links, this will definitely help, but you're right, building links to obsolete products is not a smart idea. That's why you should point most of the links to the homepage, to up-to-date / timeless category pages and / or to blog pages. The more authority your site gets (by getting quality links) and the more pages get indexed (which is where blog posts can help) the higher you might rank for all other pages on inertia of your site's authority, thus having no need to waste precious backlinks on outdated product pages.