Personally I'm not so interested anymore what the current "standard" format is. Implementing a sampler engine takes quite a time. And reverse engineering and implementing a proprietary format is a lot more time consuming than implementing an open format where specifications are available. When we start to work on a 2nd sampler engine now, we should consider what is going to be the popular format in one or two years instead. And as far as I can see this, this could be SFZ2, not nki. I talked to tonehammer yes, and yes they said they use nki, but also said that they're trying to switch to another format, like almost any sampler library vendor tries to do these days. They said they played around with some SFZ(2) capable sampler which supports the format already quite well. AFAIK Garritan already switched to a SFZ based sampler engine, Eastwest also use their own sampler engine, etc.
When we started to work on LinuxSampler (really actively), Gigasampler was the "standard" format at that point. Even though other even more powerful and more open formats were already been available, we decided our first sampler engine to be based on the Gigasampler format. One or two years later people already started to critizize that the Gigasampler format became already less popular and that we should support this and this format instead.
And I really dont want to deal with all those hacks, bugs and warts again of another over-commercially extended sampler like GSt and Kontakt. Because as you already noted, both parties just extended their feature list for promotion purposes instead of cleaning and making things stable. For us this means e.g. even though a lot of audible results in those samplers were caused by bugs, hacks or whatever, ... people expect sample libraries to sound EXACTLY the same with our sampler than with those samplers and we would have to "emulate" all these behaviors in the very detail (like we already did for GSt format). And what happens when NI decides to discontinue Kontakt? The format will be dead from one day to the other and no sample library vendor will develop any more nki libraries.