Pointedstick

In addition to the obvious answer of "because our software is really good!", IMO an under-appreciated reason is that KDE truly is an anarchic and largely volunteer-run community. As long as there are passionate volunteers, there will be KDE; you don't have to worry about it just dying one day should some big corporation pull the plug for some reason. We've all become so accustomed these days to software being disposable, but KDE really does give you a measure of longevity and continuity that you're unlikely to get elsewhere, especially without paying a lot of money for it.

What’s the best or recommended way to test out Plasma 6 RC2?

Neon Testing in a VM (or on bare metal if you're adventurous). Arch with the kde-unstable repo is good too, but that also includes a snapshot of the unreleased Qt 6.7 which introduces more bugs.

What has been the hardest problem to solve moving to Qt6?

Personally I'd have to say the large number of API and behavior changes in QtQuick that Qt 6 has brought. We use QtQuick very heavily throughout KDE, so this has required a lot of mandatory porting work, more than in our QtWidgets-based software. And there have even been changes between Qt 6.5, 6.6, and 6.7, so it's still a bit of a moving target

One of the most visible ones for me is that most common multimonitor workflows Just Work™ in the Wayland session now. There are still edge cases, but we've put a huge amount of effort into this.

Actually Plasma is generally more popular than GNOME every time surveys are conducted. However we have to keep in mind that the direct consumers of a DE are actually not the end users, but rather the distributors who package and distribute it. There are a number of historical reasons why many distributors ended up picking GNOME over Plasma including accessibility, corporate sponsorship, an easier packaging experience, and the rocky KDE 4 rollout burning a lot of trust. So what you end up with today is many distros shipping GNOME despite pent-up desire for Plasma. It's a great illustration of how you need to keep your direct users happy.

And I think that pent-up desire is being unleashed these days due to various changes in our ecosystem. Plasma is better than ever and version 5 had a much less painful release compared to 4, with us aiming to do even better in Plasma 6. We also see an increasing number of hardware vendors shipping devices with Plasma on it (https://kde.org/hardware/), who had a strong financial incentive to listen to their customers by picking Plasma over GNOME. In addition, KDE's accessibility game is ramping up hugely, and we have more robust corporate sponsorship than we used to with Valve and Blue Systems putting tons of resources into KDE. Finally, GNOME seems to be becoming more hostile to their downstreams, causing them to need to do more of their own development or else migrate to be a fork or skin of Plasma. Interesting developments.