> Consider e.g. Steam (digital video games): Prices are discounted over time because of "greed" (=> desire to sell the same product to customers that value it less than the first wave).

Steam doesn't have actual monopoly. Their position is caused wholly by competition consistently shooting themselves in the foot by either offering inferior product or just annoy the customers

For example let's take EGS:

"We will take lower cut, buy from us!"

"Ok, so that means game will be cheaper right ?"

"Of course not! Just devs get that. Also we lied in marketing and compared our pre-transaction-fees cut to Steam's post transaction fees cut. Also we didn't mention Steam lowers their cut when games sell well, so the difference is far smaller in reality"

"Sigh, I guess I might try it for that reason, how does your service looks like"

"Well it has 1% of the features Steam has and about 20% of the features you actually use in Steam are here"

"...why the hell I'd buy at you?"

"Coz we paid devs of games you like to release exclusively at our platform?"

"How about fuck you I'd just get it on Steam".

About only competitor that tried was GOG but their "no DRMed games at all" motto meant that they just don't have games people wanted.

So, Steam enforces DRM while GOG doesn't? That's an improvement!

Actually no, Steam DRM is entirely opt in and devs don't need to use it at all.