This is great news, finally the final format is here. Why, well if you search the web, 4K format will be the final stop (for a long time anyway). As moving over to 8K seems far away if ever.
And also, all the older 35mm analog films do not have more information than about 3k to 3.5k information in each frame. So all these movies that are remastered, scanned with 4K tech (or even 8k in the future) will not look any better than what ultra HD can provide, that is just a limit we can not overcome.
Over time I think people will rebuild their collections anyway as I see the price for these 4k bluray discs drop crazy fast. The technology works, players only need some minor updates, the 4k TVs have plummeted in price quickly. Because over all all, this is just minor upgrade of already existing technology so they do not need to get a lot of money back, they already got that with the standard bluray format.
What I do not like is the fact that you can not re-scale older formats on a lot of technology, old SDTV looks way crappy on a 1080p not to bad on 720p, but on 4K. Scaling 720*576 in a 4k screen will just be totally horrible.
Would be nice if they had made this into standard on players, TVs so that you can scale the picture source to a good size in relation to the actual pixel screen size. At least then you could watch older stuff on modern equipment without it looking to bad.