Well corporate rights are a bit different and probably wouldn't overlap with promissory estoppel in the hypo you made up. The closest that I can think of is corporate purpose. When filing the articles of incorporation with the state Secretary of State (when you first create your corporation) you have to state the "purpose" of the corporation. Generally these are really broad and just state something like "engage in all legal business"; however, a corporation could state a more narrow corporation prupose. E.g., as you suggested, "sell beauty products over snail mail and not to use this new fad 'the internet'").

If in this scenario, the corporation were to then venture out and go outside the scope of their corporations purpose (through either engaging in business in direct contradiction to the corporate purpose or by going very far outside the scope of the stated purpose) the corporation would be engaging in "ultra vires" behavior and the corporation's directors would open themselves up to liability from shareholders. [Although you can change the stated purpose of a corporation at a shareholders meeting]

So while Promissory estoppel might not apply to the corporate structure, there is at least one narrow mechanism by which a corporation could be prevented from engaging in behavior it explicitly swore off.

Now, as per how promissory estoppel might apply re extended periods of time. I haven't actually done the legal research to state with any actual authority (aka, I might well be wrong about the specifics) and as I stated this isn't the area of law that I practice. However, my "best guess" would be that it wouldn't matter in this particular case.

The thing is that people continue to rely on it (hell, when I picked up my playset of Trops and the candelabra's to play 12-post even I relied on them not suddenly becoming worthless due to a mass reprint) and while the nature of the US marketplace has changed with the internet and WOTC/Hasbro has gotten much larger, the nature and purpose of the promise by WOTC hasn't changed.

The long story short though is that WOTC has DEFINITELY done the proper legal research and concluded that the hundreds of millions of dollars they could make by abolishing the reserve list and literally (figuratively) printing money by selling successive FTVs with 1 piece of power / 1 dual land + jank is just not worth it due to the certainty of the ensuing lawsuit.