Undermines traditional journal publishing

This is a common misconception. Repositories aren't replacing publishing. They work alongside publishing (acceptance by a publisher is normally the criterion by which we accept material into repositories). Libraries can't subscribe to everything, so depositing in the repository ensures the institution has a copy. Also, if journal subscriptions are cancelled in the future, the library might lose access to back-copies. This didn't happen with print copies but is a risk with online only-content.

Take opportunity to talk about open access models and issues. An overly evangelical approach can be counterproductive. It is important to listen and respect other points of view - go for the long game.

Generational difference – younger researchers may be more willing to engage.

Be aware that publishers are powerful lobbyists and can be very influetial with some academics.