Icewind Dale was released in 2000, Skyrim in 2011. Enhanced Edition IWD was released in 2014 but that was an enhanced edition, given some polish etc. It was already a 14 year old game by then.
Neverwinter nights was a game from 2002 and uses 2rd edition, IWD uses 2e infinity engine mechanics. (IWD2 was also 2002, which also uses 3e rules. The only infinity game to do so)
It was not meant as a sequal, but a seperate series using the Infinity Engine, showcasing and focussing more on the combat rather than storytelling.
Regarding sales, you cannot compare them directly 1 to 1 without taking context into account, really. Games becoming more popular, gaming studios becoming more popular and getting more "brand recognition". Comparing DOS2 sales to older games like BG without including their earlier versions? Divine Divinity, Beyond Divinity, etc. Then there is the context of other games on the market in those years, as well as Enhanced Editions, Definitive Editions, Diamond Editions etc.
There is a lot of speculation and claims that are simply incorrect in your arguments.
