"I did wonder if it would be better if people could not merge information from other trees."
Even if copying from other Ancestry.com public trees were made impossible or difficult, the same old junk would remain and material such as copied GEDCOMs from elsewhere could still be uploaded. This internet tree error rate has existed for a couple of decades, and will continue as long as there are places to put such material.
Continuing efforts to demonstrate and teach how to do actual research, as well as how to draw conclusions from evidence, will have an impact on those willing to learn. But the majority, as in most human endeavor, will not be willing to put forth more effort than required to meet whatever the standard of their objective may be.