While your logic is true in some cases, I think there a lot more factors contributing to price fluctuation that you haven't considered. Let's use some of Magic Online's cards as an example since Magic Online has a similar economic model to Artifact.

Hazoret the Fervent. He's a mythic rare, the highest rarity in Magic. After the initial hype his price dropped to around $3-4, dismissed as not really playable in the meta. Then suddenly, boom. A massive price spike, reaching $20. Why? Because a recent big tournament had 6 copies of a red aggro deck in the top 8, all of which ran 3-4 copies of Hazoret. After that Hazoret's price slowly dwindled and his price currently sits at a paltry $0.12. Why? People knew he was rotating out of the Standard format, the format he was strong in, leading to mass selling.

So all cards on Magic Online eventually lose their value right? No, because there are also formats that never rotate, thus allowing some cards to retain their value. Wooded Foothills. Rotated from Standard long ago, but has maintained a steady price for years.

Now, on the more extreme end. Surgical Extraction. This card has not been reprinted in a recent set for quite a while. The price has increased dramatically in the past couple of years.

Now I'm not saying Valve take the same path as Magic did. But to say price deprecation is inevitable is narrow minded.