While I see your point and I agree that the general "downgrade"-issue (in all games and for all developers) should be handled more openly and honestly, I can not understand somebody who's upgrading his PC on the basis of a trailer.
I mean, it's not like it takes ages to build a PC, does it? You can order the parts, wait 2-4 days, build it all together and get going. It's kinda frustrating that we're not getting PC reviews now, I was kinda basing my own upgrade decision on that. But however, I'm going to wait for launch and decide what and if to order any upgrades. And even if you can not wait a few days post release until you want to play, you should never, ever upgrade solely based on a trailer, screenshot or developer announcement.
Even if the developer has the best intentions it's entirely possible that they present you with a vision of what they believe it to be - and after a year or two later they're simply getting to a point where they have to admit: It's not possible. That does not have to be a "let's just lie to all our customers"-approach, sometimes things just don't turn out the way you wanted them to. However, I agree that this situation should be communicated way more actively: "Look guys, we're working hard on this but some things turned out to be impossible while maintaining half-decent frame rates. Sorry." On the other hand I can completely see why devs aren't doing it. The internet can sometimes be a very nasty place and seeing some reactions in games communities (and on non-games communities too, like the public shaming we regularly witness on Twitter) I can totally understand the reluctance to approach this topic in a more open manner.