I agree that a universal canvas fingerprint is better in principle, but practically who is going to write a script to search for all visitors who only differ by their canvas fingerprint and then identify them as one browser because the fingerprints are non-standard?
Practically, it requires little more work than creating a canvas fingerprint framework itself! If someone puts in the effort to write a framework that tracks you via canvas fingerprints, it’s little more work to add to the script with another one that performs a simple diff to find people trying to evade it.