The problem then is that you have a dead string and no idea which one is the
culprit, which is the way they used to be 30 years ago. With the
introduction of the "wire" at least you have a chance of finding bad ones
easier.
Of course, that's assuming that one is using miniature light bulbs.
Thirty years ago, many people still used the larger light bulbs.
I've never used miniature light bulbs on Christmas trees, always the
colored bulbs that are about an inch and a half high, from base to tip
and about 3/4" around, wired in parallel... grew up with those, and
still use some of the same sets of lights... the older ones were much
better made and easier to string on the tree, since the wires weren't
as stiff. The little lights were only used in the windows, which they
could be Scotch Taped to... and no blinking lights! (except for the
red bells that made a tinkling sound when they blinked on and off) It
isn't that often that the little miniature bulbs burn out, but when
they do, it only takes a few minutes to use a battery-operated bulb
tester to check all of the bulbs on a string.