https://gs.statcounter.com/os-market-share/desktop/worldwide

As you say, Qualcomm is probably the closest to being competitive, but there is a lot of work to do to catch up. Nvidia is trying to acquire ARM, so they seem to be interested in moving into the space. Samsung and AMD are now working together in ARM processors, so they are another player. Intel used to have an ARM presence via their DEC acquisition, but that was sold to Marvell about 10 years ago. Marvell might move into the space. There are also some ARM startups, some of which were founded by Apple engineers. Lots of activity. Intel is probably the biggest loser in all of this. Sad to see that. They will respond, but it will take time.

At any rate, in terms of desktop market size, Windows is the biggest (76%) and getting a chunk of that processor revenue is a big enough payoff to warrant the required investment. Doubling or even tripling Linux desktop share is comparatively small potatoes. Microsoft is mostly agnostic so they will encourage cannibalizing X86 Windows in favor or ARM Windows rather than lose market share. I think we are going to see a big uptick in ARM Windows investment and product announcements.

Perhaps - if it makes sense, cost/performance/power-wise to put a core like that in an Android mobile phone. I guess it would? But in the next 5 years?

Remember that anything Qualcomm makes will be optimized for mobile phones and only mobile phones. They won't waste any die-area at all on anything that isn't required by the Android phone market. The Android phone market is the only market for these chips that sells enough units to pay for their design, and it's fiercely competitive.

Especially with ARM's own designs improving so much and Samsung abandoning their own under-performing designs in favour of ARM's, Qualcomm is going to get a lot more competition in the next few years.

Anything they put on those chips that makes them more expensive or use more power than competitive chips is going to cost them design wins. No way will they sacrifice 10% of their mobile market for some pie-in-the-sky, maybe-maybe-not ARM laptop market that doesn't exist and will depend on lots of theoretical buy-in from Microsoft/Redhat/Canonical/Adobe/Lenovo/Dell/etc.

The legacy support of closed source software windows has is working against them atm.

That's pretty outlandish. Try saying that to someone who uses Excel or After Effects or Photoshop for their work. A performance linux-based arm64 laptop has everything working against it that a windows arm64 laptop does, and arguably even more.