"1. How long live one version KDE ?"
kde developers release this information, they say when a beta is being tested, when it will be released and what features will be added in the next version, when the new version is released, you can keep the old one for some time, we did that with kde 4.6.5, we used it for almost a year and a half but depends on multiple factors
"2. Will it be possible to make stable KDE ?"
pclinus always uses the most stable version released, we don't use the latest just because it is the latest
if you find problems with the current version of kde you can try to fix those problems or use a older version from another distro that doesn't do rolling releases and makes you reinstall each 6 months or each year, that way you don't have to worry about updates because there isn't any
kde is just a base, it can support some tools, extensions and apps but not everything can be supported forever
the idea is that if a new version is released, the developers of apps and tools related to it, verify if something has to be updated or fixed to keep them working with the new kde release
it is the same as in firefox, they offer a base and tools to develop extensions, addons, themes, skins and plugins but if the developer leaves his creation abandoned and outdated, firefox is not the one to be blamed if it becomes insecure, obsolete and stops working
when kde 4 was released, pclinux waited a long time before adopting kde4, while the new kde4 was stable enough for us, that took almost 2 years
when we began installing it, many users complained about some apps not working as it should and others not present at all
some were direct kde development team fault but others, basically were abandoned by the original developer
one was really important to many users but the developer never did much to try port it from one platform to another so the app was abandoned for almost 4 years, i ignore his current status, can't even remember the name
possibly quanta plus but not sure
think on kde as a base, we use the most stable base available at the moment and we keep updating or fixing what needs to and keep up with the linux environment that is evolving considerably faster than other platforms
"KDE also featured to work well on older hardware
( Full Scene Anti-Aliasing
Transform & Lighting
PhysX, and more... )
This is still a software problem."
part of this is depends not on kde, depends on the companies releasing or not tools, information and drivers for their video cards, linux works with what it has or what it can while trying to support new hardware, taking advantage of new features and trying not to abandon hardware when it has been abandoned by the manufacturer along time ago
linux in general tries to cover a really big market, desktop, laptops and portable devices(that is now a even bigger market) and that surely is not a easy task