[pkasting]

Rust as a language was significantly less mature a few years ago; I think that's the bigger factor here.

It certainly doesn't hurt to do compelling things in Rust in a browser engine, but doing compelling things in Rust in some other project entirely would also be motivating.

To look at it from another angle, Mozilla itself had a reason to try to tackle some problems with Rust. It didn't need a competing browser engine using Rust in order to move forward with that plan. And the plan wasn't "well, we'll try this because we can somehow fall back on a competing engine if this doesn't work". So if your argument were true, I don't see how Mozilla could have moved on this either.

In the end people do things because the potential benefit justifies the costs and risks, and having a competitor do something is not the only way of determining potential benefit.