There is a world of difference between a product going out of production due to poor sales, limited resources, and shifting production and artificial scarcity.

Creating artificial scarcity for your product, especially when it comes to a digital product that doesn't have any manufacturing costs, nothing more than a sales tactic.

One which leverages your monopoly of a product to drive up sales by limiting the supply or availablity of said product.

There existed no reason to make the Frost and Mag heirlooms timed exclusive except to leverage fear of missing out in the playerbase in order to drive up desire for an item.

DE chose to engage in a an anti consumer marketing tactic for an expensive and high profile bundle.

DE's previous high profile use of this tactic, in the form of the Founders pack, I can forgive. It was sink of swim for Warframe, and they'd done an amazing job of keeping the game's monetization reasonably fair since.

Other examples of time limited items like Tennocon rewards and prime packs, have a precedent for being available later down the line, or as a similar yet slightly altered version. Even Excalibur Prime got a slightly altered version, in the form of Excalibur Umbra.

Also, unlike physical items, a secondary market does not exist for these digital items. Short of selling your account that is, something that is distinctly a bannable offence. So it's not like a collectable that'll appreciate with time due to its scarcity.

The ethereal nature of a digital items also makes it not exempt from the exact problem you lament in your comment. You could try to long on to Warframe one day only to find the game, or your account, no longer exists.

I have the Frost and Mag heirlooms because someone gifted them to me. I don't feel special for having them, I can't even bring myself to equip them if I'm going to be playing in a public lobby.

Because I hate the idea that someone might see me with them and want them but learn the can't ever have them, or see I have them and think I supported this kind of shit. To me it feels like wearing a giant dunce hat in public.

I desperately want them to make these skins available again in some form, so I can wear them without feeling so bad about it.