It's true that less creative work would be produced without the expectation of a financial return on that investment. There's some margin of writers, filmmakers, and musicians who would instead choose to dedicate much of their time to more profitable activities, such as machining, driving an Uber, performing surgery, or flipping burgers, because they aren't independently wealthy and aren't willing to make the sacrifice of eating brown rice and lentils for every meal in order to dedicate more time to their art. And the ideas that would never be born are a loss to society, and not a trivial one: we'd still have Wikipedia, surely, and Collateral Murder and Beej's Socket Guide, but not Welcome to Chechnya, Game of Thrones, or Westworld.
But we need to weigh that loss against the vast devastation inflicted by copyright today.
Our cultural heritage is being burned because it's illegal to preserve it.
Innovators who create preservation tools get prosecuted for inducing circumvention of access control measures.
People with rare diseases are cut off from access to the research literature that could save their lives.
Our private communications through Facebook are being automatically scanned for forbidden information, a censorship infrastructure beyond anything Mao or the Nazis could ever have dreamed of.
Books we "own" evaporate from within our homes when Amazon decides 1984 is still in copyright in our country.
Every new internet startup needs to have an adequate censorship strategy to enforce copyright with, or Viacom will sue them for a billion dollars, as they did with YouTube; this strategy is incessantly used to silence criticism with wholly groundless copyright claims, including by the infamous Dr. Stacy Makhnevich: https://www.wired.com/2014/01/internet-companies-care-fair-u....
YouTubers are constantly extorted by false copyright claims; using Creative Commons licensed recordings of public-domain classical music on your video is a near-guaranteed way to get multiple copyright strikes.
Printer makers lock out competitive ink cartridge vendors, extorting ridiculous ink prices from their users.
Farmers are forbidden from repairing their John Deere tractors.
NFL telecasts used to purport to forbid you from writing descriptions or accounts of football games.
Only the biggest companies like Google can safely use free software like Apache Harmony --- and even Google almost lost in the Supreme Court.
Internet streaming music radio was a promising new medium before being strangled in its cradle by copyright.
And, of course, Aaron Swartz committed suicide after a baseless copyright prosecution.
Reducing the term of copyright and punishing the most egregious abusers (like Makhnevich, John Deere, and the NFL) might reduce the magnitude of the problem, but it doesn't change the essential conflict: copyright attempts to reserve the right to preserve and disseminate our intellectual heritage to copyright holders, while the great public benefit of computers and the internet is that they make it trivial to preserve, disseminate, and extend that same intellectual heritage. Fundamentally these are not compatible.
None of this has anything to do with plagiarism, which is a question of giving credit to creators, except in the accidental sense of being lumped into a single body of law.