> I strongly suspect that the people who can't adapt are having difficulty because they are not programming with the new syntax full time. Practicing at night while the majority of your time is still spent with some other language is not the same thing. This is also true for learning human language and is why immersion programs are so successful.

I think this is spot on. I have a minor quibble that can't adapt ought to actually read find it difficult to adapt for the very reasons specified--it's not a matter of can or cannot, but a reflection of how much time one is actually able to invest in learning, like a human language. Random, sporadic investment into learning French vocabulary won't help much in improving one's ability to actually speak French. Much will be forgotten that way.

Immersion is definitely the best model for picking up any new [programming] language. Each language I use with measurable proficiency--Python, Objective-C, C#, JavaScript, French, English, etc.--is in that list because I actually took on tasks in which that was the language I had to use. Each language I've messed around with on the side for fun/experimentation/curiosity--Mandarin Chinese, Lisp, Haskell, Java, etc.--is barely worth mentioning.