Earlier quoted context omitted.
unless the thing you know everything about - like say FAX - is something that keeps expanding all the time, then once you know everything about it you should be able to structure your time to learn a fallback you can switch to if the necessity arises, which is basically what I did when my specialty was no longer in great demand.
I'm curious... tell me more about this. What specialty did you have? What did you switch to?
Now at this point if I wasn't such an artistic punk type I would have gone off to private industry and worked in banking etc. as the Danish OIOXML / Efaktura specialist doing consulting work for a few years at a few hundred dollars an hour, instead I got enticed by a friend to go work at a company that I hated for the next couple years because we were going to build a media management product we had tried to do a few years before but this time to do it right!
After which there wasn't such a big oioxml efaktura thing anymore but I did still have XSL-T so I did that for several years at Thomson Reuters and then when downsized from there nobody used XSL-T anymore but I had during these years developed my backup search technologies and frontend skills.
The last 5 years I've been mainly frontend with some 3-4 months every few years doing search stuff.