It’s wasteful, immediately hazardous to any crews, and eventually (if not also immediately) harmful to the environment with any wreckage and other pollution that may be produced.
Interesting, I interpret “Europe must embrace a cultural shift towards speed and innovation” in almost exactly the opposite way.
Ariane 6 was essentially an outdated design before it even launched. All of the major American and Chinese launch companies are operating or developing reusable rockets. The launch startups which wrote the open letter are some of the only European organizations actively pursuing reusability, something Arianespace has ignored for far too long.