1. On the issue of TN panels, it's been pointed out before there reality is exceeding perceptions. When Adam breezily says he has been burned by past experience, I'd appreciate if he would name those monitors. 
There are 4K TN panels out there who have really good color reproduction, something that many picky review sites like Sweclockers or Pcper.com have all pointed out. The vast majority of TN panels, however, are not good. 
And the ROG Swift had such a good TN panel that Sweclockers said it beat several IPS panels in their (very thorough) color testing. 
So the reality is that it does depend. And I can't speak which panels were recommended in the past or whatnot, but it's a good rule of thumb to look at what the pros are saying, not random members. And the pros are saying that some TN panels are in fact doing really well, even if they are in the minority.
2. As to the issue of fast panels etc, there aren't any technical issues preventing manufacturers from putting out 1 ms IPS displays. It's more expensive but it can be done. 
The thing is that most of the people who get an 4K IPS display are professionals as of now and not gamers. How many of them need very fast panels? By pushing up the price, you price out a lot of other potential buyers out of the market, so the TN/IPS divide persists for now and will continue to do so until GPUs get more and more powerful and 4K panels naturally fall in price.