By proven itself over many flights, I wasn't quite referring to reuse, but rather just that by the time it is flying people - unless it's something that Congress couldn't care less about killing people with (like SLS) - the class of rocket will have flown many times.
Also, reusable rockets were not considered worse. It was obvious to everyone that in a technical sense, a properly designed reusable rocket was better (ie not the Shuttle, where most of its design was dictated by missions it might be required to handle).
What other companies/agencies disagreed on was if they were economical. For example, ULA expected to have to fly a booster 10 times to break even on it for reuse and Arianespace similarly believed that they weren't having to launch often enough to justify a reusable rocket, arguing that if they went reusable they'd just have a bunch of people with nothing to do most of the time.
Also, reusable rockets were not considered worse. It was obvious to everyone that in a technical sense, a properly designed reusable rocket was better (ie not the Shuttle, where most of its design was dictated by missions it might be required to handle).
What other companies/agencies disagreed on was if they were economical. For example, ULA expected to have to fly a booster 10 times to break even on it for reuse and Arianespace similarly believed that they weren't having to launch often enough to justify a reusable rocket, arguing that if they went reusable they'd just have a bunch of people with nothing to do most of the time.