When you make statements like this, you need to qualify the goal of whom. I've run Linux on my desktop since '95. It's not my goal to get mainstream adoption - mainstream adoption is irrelevant to me. The same is the case for a lot of people, and quite a few Linux "desktop distros".

Specific distro's might have mainstreaming the Linux desktop as a goal. Some might have failed. Some are still trying.

But there's no single goal here. Nor even a single vision of what a "Linux desktop" should look like.