No, they haven't. They don't know or care. Even those who do know - like myself, you, and the other AT members - likely don't base GPU purchases solely on proprietary features. Those that do are free to make their own choice, but I've been computer gaming for over 20 years and I have never encountered a game that I wish I was running on the competitor's GPU. Outside of the rare case when a game is crippled/broken on one brand, there is no meaningful difference between gaming on one brand of GPU over the other at the same performance level.
This is almost entirely about developers and publishers wanting to offload development costs on to Nvidia and AMD. Nvidia and AMD oblige because they want the exposure, the press and marketing, and they are hoping to lock customers into their brand.
We are 10 years away from Nvidia and AMD just having to supply their own complete game engine to devs. At least, we were before the main engine techs could be licensed on the super-cheap.
I have no problem with Nvidia or AMD developing gaming libraries specific for their GPUs. I do have a problem with developers and publishers using GPU specific libraries with licensing agreements that lock-out their ability to work with the other.
You're conflating two entirely different things.
Developers implement it because they're getting free labor or help or equipment from Nvidia. The implementation affects consumer perception of Nvidia. I'm not even really sure what your point is. Half your post isn't really disagreeing.
I do agree that developers are putting in features because they want help or more features they can sell about their game. But fact is, people see this stuff and it affects their buying decisions. The idea that consumers don't care is purposeful ignorance. The typical PC gamer isn't some blind idiot. They follow the popular tech "news" sites, all of which report on stuff like physX/FLEX, G-sync, Gameworks, Shadowplay, etc. They watch Nvidia's demonstrations and events. They pay attention to news about upcoming games and what sorts of features they have. They absolutely care, and they absolutely pay attention. You think the people waiting for Witcher 3 didn't pay attention to the Hairworks demos or people didn't pay attention when Borderlands 2's devs started showing off Physx? People care and people want to be able to use those features. When Nvidia's name is attached to those features you can bet it affects people's buying decisions.
You can what you like. But these proprietary, locked down features are a big part of Nvidia's marketing campaign and their reputation as innovator. The only thing people don't care about is whether something is open source.
NVidia being the dominant market leader is not a recent development. They have had over 60% market share since the end of 2011. The degree of dominance has increased recently, but not the fact that they are the dominant force which allows basically everything they add to their cards to be instant market leaders. When Nvidia released Gameworks in Q1 of last year, they commanded 65% of the market. Now they are in the low 70's. Not a major shift, but one which has far more to do with the release of Maxwell, while AMD has released nothing since then.
http://www.anandtech.com/show/8446/the-state-of-pc-graphics-sales-q2-2014
AMD was up from 35% in 2013 to 38% in early 2014.
http://www.gamesindustry.biz/articles/2015-04-17-amd-lost-USD180-million-and-gpu-market-share-in-q1
By Q4 2014, it plummeted to 24% and dropped even further in Q1 2015. Not sure how much but probably around 20% by now, or 80% for Nvidia. It's probably dropped even further after articles about Witcher 3 gimping AMD cards. It's a significant change. And AMD was certainly in a better position in early 2014 with almost 40%. Also, during the Never Settle campaigns AMD showed that they were able to get a lot of devs and titles on board with GE. They definitely weren't being ignored because they had less market share.