very few linux users are likely to argue about the merits of OSX, they are plenty (stable, nice GUI, apple marketing push). however, making the assumption that OSX will kill linux is wrong for several reasons:
1) linux is multiplatform, you can run it on sparc, ppc, or x86
2) linux runs on multiple vendors hardware, OSX only runs on apples hardware (which is prohibitively expensive
3) linux has a very broad developer base that hasnt disappeared since OSX was released
4) currently there are more applications for linux than OSX (no knock on OSX, just there isnt yet)
now id love to run OSX, but that would require me shelling out my money for OVERPRICED apple workstations. thats whats gonna keep OSX from becoming the windows/linux killer. if apple released an x86 version of OSX (which shouldnt be too hard), it would carve a massive chunk out of redmond, but apple isnt going to.
now i asked a question a while ago to an Apple salesperson at Frys Electronics a while ago. I asked him why apple hadnt pushed products into the x86 marketplace. all he could say was that intel/amd werent powerful enough to run Aqua, specifically sighting architectural superiority of G3/4s. now i had to laugh at this as i pointed out that OSX is running xwindows (am i wrong?) which i run quite well with my nvidia graphics card on linux. he had nothing to say to that accept stumbling about the advantage of motorolla's G4s as a RISC processor over the overly complex x86 isa (which i generally agreed with). but then i asked him about fair benchmarks between the two and we decided on Apache, Sendmail, and Quake3. well guess what i pointed out to the apple zealot? all of those run equally well or faster on x86 linux. he seemed pretty incapable of convincing me of apple's superiority in hardware.
but i never argued any of his points on software. OSX is a great product, its fast, its easy to use, and its stable. apple always gets good developer support for audio/visual developers, which is a major plus. and they finally have a BSD core, which means that nearly everything that will run on bsd, will run on OSX, which is great decision.
so IMO, apple wont move beyond a niche market until they drop their hardware division.