A proper OO support makes difference for some use-cases. That Serenity OS guy is building a web browser and recently spoke about it. Game developers also complain about the lack if it in Rust.
At least delegation (IIUC closer to concatenation as defined in [1]) is currently being implemented, but I can't say how much weight it can actually carry for the OOP efforts.
[1] "On the Notion of Inheritance": https://users.csc.calpoly.edu/~gfisher/classes/530/handouts/...