Long term, it pays to be multi skilled. Otherwise, you may end up stuck maintaining ancient legacy systems with little opportunity to change jobs. Just ask any COBOL programmer. It can be comfortable but is usually unsatisfying.
In the short term, being a master of a new and popular technology can be very lucrative. That is until the market shifts or the number of master level programmers increases. This is essentially what happened during the dot com boom and bust of the late 90's.
It is probably best to focus on one larger area, like data or UI, but not be tied down to a narrow set of technology, like a particular framework.
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