Earlier quoted context omitted.
This still restricts you from getting many of the jobs that require narrow specialization. Specializing on the fly only works to a certain extent and only if you get the opportunity to do so in the first place.
In my experience, "narrow" professionals tend to heavily overestimate what they bring to the table, which makes them not very hard to outcompete.
Or several domains if they are simple enough. Like, the typical needs of a web product are simple enough that you master how to do the database, back end and front end. You wont be able to do the back end or database at Google scale, but you can master doing them at typical scale and I'd actually argue that Google scale is a different domain entirely with a different set of skills, you need a different specialist to handle that problem and he wont be able to efficiently solve your small scale needs either.