Warren wrote:

I am not 100% sure that it is the line manufacturer's fault though.
I mean they are the ones making the lines that way, but could it be
because of how modern rods are manufactured and sized?

It is precisely (partly) because of that. Also because they really want
to convince fly fishers that they need a different line for fishing for
pike, for fishing for bonefish, for fishing for trout, for fishing for
bass, for fishing nymphs, for fishing when there's wind.....

What if a
classic 4 weight line doesn't work on modern "4wt" rods?

You put a 5 wt on it and you blame the rod manufacturer for seeding
confusion by producing something that isn't as labeled.

What is a
line manufacturer supposed to do?

Produce lines that match the label on them. Period.

If the rod manufacturers aren't
following the standards, why should we blame line manufacturers who
adapt to the rod changes?

Why can we blame just one or the other? Rod makers create chaos by
producing rods that don't load "optimally" (purposely in quotes, and
understanding that other factors such as taper, length of leader, size
of fly, etc., come into play) with 30 of the matching line wt out. This
chaos is good for sales, bad for both the casual and the average fly
fisher.

By saying, in effect, "Now we can play not just with taper, materials,
etc., but also with the weight that should correspond to the labeled
line WEIGHT," the line makers can further jerk the consumer around and
jack up sales just like the rod builders.

But maybe you're right.... just a sign of the times. Nothing can be
done. Let's all just roll belly up.

JR