Something like yesterday or the day before I was browsing for games and found some nice old RPG games (like Sacred and Sacred 2) on sale for a whopping 90% or so.
Now, one/two days later, I decided that I want to buy them, but it seems the sale has ended! Price has gone up this 90%. That's too much.

How do I know when it will be on sale again? (If ever)

They have sales here every week. They also have quarterly summer/spring/autumn/xmas sales. Just wait, eventually they will come on sale again.

Since it is addressed here... I have this moral dilemma for some time now. Maybe I can get an honest answer. Do we really affect the developers and GOG when we buy discounted games? It would be logical to assume that discounts are kept within reasonable limits, so that developers and GOG do not suffer from them.
Will I be able to play discounted games with clear conscience ever again?

But what happened, that would cause you such torment?

Its a simple business transaction, the sell something you buy something end of story. Stop having the daft idea that these companies/publishers/developers are anything more than a commercial venture. Much the same as if you goto the store and buy something on special offer there, simple business transaction.
Much like is endless dlc, micro transactions, and all the other things are not moral questions, but business. If you don't like those things, don't support them by buying into it. There is no moral on just business.

I don't have an answer for you. So here goes! ; )

There's no direct coercion on a publisher to place a discount on their titles. There are, however, market forces. The marketplace we're participating in - and helping to mold - has developed into one where large discounts are the norm, often within a couple months of release.

While I don't quite agree with nightcrawler's assessment that these companies are nothing more than a commercial venture (I sincerely hope my own customers don't think of my business that way), I do wholeheartedly agree that if a consumer does not like certain things about the marketplace then one should not support those particular practices.

That said, will the publisher notice? While 'voting with one's wallet' is certainly a valid approach, that sole act does not tell the product provider why or why not you voted the way you did. If it's an issue that really concerns you then a polite message to the provider will at least give them feedback beyond sales numbers.

When it comes to old titles that really weren't getting any sales volume to speak of since long after a product's release, and for which publisher support has all-but-ended as the product has matured into a stable state, I have no great qualms about buying on big discount. Yeah, I do wonder how they make any money after the financial transaction fees are taken into account, but there must be at least a little bit of payback else they wouldn't agree to a 90% drop. It might also be that the 90% discount is a "loss leader", a money-losing enticement to get you to look at the other products that DO provide some profit to the store and publisher.

Well, I consider us a big gamelovin and drm-free-lovin community. It feels wrong to bring the whole drm-free industry down by pushing prices down to zero. Instead we should support the developers by buying games at full price. In theory that is...

This has been discussed a number of times on here, but the bottom line is this: people will buy only at a price they feel is fair for them. Companies will sell only at a price they feel will make them the greatest return. This point is somewhat fluid, but if GOG and/or publishers/devs don't feel they can make a profit, then they won't sell at an ultra low price. If you don't feel comfortable buying at the deep discount, then wait until the sale is over and buy at full price.

I haven't played 95% of the games I've bought over the years (1378 games on GOG alone, plus games on Steam, Humble Bundle, retail games), and most probably I will not be able to play them all in my lifetime.

So it kinda evens out. Sure I have bought most of them on discount, but that's fine as I won't have a chance to play most of them. It will be even worse if you consider all the games you have played only partially, not all the way through to the end.

Plus, remember that when you are buying a game on sale, it is a price that has been OKed by the publisher and GOG. So no, you are not exploiting them, as you are paying the asking price.

That's an interesting idea. Which developer said that?

I used to think that it doesn't matter during which sale I buy a game because next time it will have the same or better discount. However for example Bethesda reduced discounts recently. Same with some old games like Zeus & co which used to be 80% off and now they were only 40 during the recent sales. Thus my strategy is this: if I want a game and is at least 75% off - strong buy. If it's 60-66 and I badly want it, perhaps buy. Over 90% buy even if I never might play it (OCD).

I remember that dev post, but don't recall which it was.

Like GR00T, I don't remember what game's release thread it was in (and I'm too lazy to go crawling through the forum looking).