I think you are missing a large point, if you have competent admins, and I stress admins, (having one super-hero admin is a really bad business decision and you deserve your whitebox+Linus solution to fail hard when your admin has some life altering event), then you can totally deploy Linus tree into mission critical places. Google, facebook etc are prime examples of this, even though google might be a few kernels back they are getting closer to mainline. However if you have management or admins who like to spend time with their kids/families then you have to have some sort of support place you can call and someone you can blame. Now the company providing that service cannot provide bespoke Linus kernels every 3 months, it just isn't practical.
Enterprise distros also have a whole bunch of certifications (government, application) etc that it isn't feasible to redo every 3-6 mths it can takes a year or so to get some of them finished.