That's the big question, with a universal application: that tension between the fact that knockoffs have a diluting effect on the original, and the idea that if your idea can so readily be copied it's perhaps not worthy of protection.
The general rule is you are to ask the individual you are copying if you can copy them in which case they will say no unless they sell the trick, or you can argue that you independently came up with the trick, and the person agrees with you.
Copying is always considered "theft" even with attribution unless there's specific permission.
There is a case where a performer does magic to the same song (Shape of my heart), but with different actual magic performance and it is considered copying.
*Edited to improve my tone, the person I responded to deserves good faith and seems quite reasonable.
I spent most of my life in the opensource and academic communities, where free sharing of techniques and knowledge was not only commonplace but encouraged.
It's very sad that much of the magic world is so anti-sharing and so jealously guard the knowledge they possess from each other (unless you can pony up their asking price).
This aspect of parts of the magic community is a huge turn off for me.
Much of the magic community is a huge turn off to me, but it's an enjoyable hobby that I have a lot of interest in.
Sort of like the way the BSD license works. However if credit is given and the new effect is sufficiently unique/innovative, then no objection is reasonably made.
In today's world, these age-old effects or gimmicks are popularized by various working magicians, some of whom make a name based on them. And once in a while a completely novel approach is invented.
Most new effects are the result of borrowing. A card trick may use a lift invented 100 years ago and a bit of verbal patter similar to something invented last year. The respectful magician will give credit where it's due and will still be respected if he has truly innovated.
If there is true innovation no magician will begrudge him the use of the borrowed elements, unless of course they make up the bulk of the trick.
The magician's creed is closely correlated with the steps needed to gain the respect of other magicians.
You're focusing heavily on borrowed elements from long ago. Sure.. No one cares. However take something from a current performer and it will not be respected.
I do agree that the magician's creed is very much about the steps needed to gain respect of other magicians.