Like 3 of the 5 games I mentioned I bought for full (or close to full) price or via pre-order before they went on sale or ended up on PS+ for completely free within a couple of months. That's just my offhand personal experience, but I am seeing this as still being the same.
Founders and Ultimate pack buyers will still get everything they paid for. 4 DLC's + character for founders, or skins for ultimate. Now for 2 days steam players can claim the first 3 DLC's for free, which is $30 tops, but they have been putting Alien Myths on sale lately as well. If they want to buy all of the skins in the ultimate edition, they will have to buy quite a bit of Mercury Points to do so as Ultimate gives an overall discount to those. Plus, would still need to buy Council Apocalypse.
Founders is a little different since a lot of what that came with was in-game fast unlocks of expensive characters, that the rest of us had to grind hundreds of hours for. I also don't know the price since I started after it ended.
Just want to put into perspective the Ultimate Edition: $65 for 4 campaigns and 5 high priced skins. I personally feel these skins are all way too expensive, but for the purposes of what you paid for, the skins were well advertised and their prices were available in-game. The total cost of those 5 skins is 6720 Mercury Points, with the best MP pack costing $20 for 2500 points. So the skins alone, for someone who wants all of them, would need to buy at least $50-$60 worth of points, plus whatever campaigns. If someone gets these free steam campaigns, and then buys Council + the skins, they will have paid more than Ultimate Edition buyers for the same content. They did not pull a fast one on you, as you know that the 4 campaigns included in Ultimate are $40 not $65, a lot of what you get from Ultimate were the (overpriced) skins.
For reference I buy Warframe Prime Access packs on occasion, and all of that stuff is eventually made available in game, or sold for cheaper in the future after vaulting/unvaulting. I still buy Prime packs when I like what they have to offer, because I still think it's a good deal. I like the cosmetics, and the Raiders Ultimate edition was mostly about the cosmetics.
I'm not saying that you can't feel bad that something you paid for is currently free for 2 a day promotion. But you paid for much more than what is currently free. Calling what they did "insidious" when doing the math on it all is an unfair claim.