Is it worth the upgrade?

I have an Alienware Area 51m gaming laptop, with an i7 9700 and an RTX 2060. I want to upgrade the CPU to an i9 9900k. If I were to build a gaming desktop in the next 1-2 years I will use the i9 there.

Bring little.

Do you want to swap the CPU in a laptop? I don't think that's possible. Especially since the jump from 9700 to 9900, if you don't get the processor for free, will probably not result in a good cost / performance ratio.

Especially when it comes to gaming, the CPU does absolutely nothing.

The laptop has a socketed CPU

The CPU in a laptop can't be changed.

They are either permanently soldered or not even available on the open market. In addition, the cooling is specially adapted to the application.

You can also order the right tower cooler at the same time (sarcasm end).

With the alienware area 51m you can exchange the cpu, is a standard desktop cpu.

The i9 has twice as many threads (16 vs 8) and I would use it later in a desktop with a more powerful graphics card.

Don't just play games, I also sometimes do mixed reality recordings, which is where the CPU quickly reaches its limit. If I then buy a 3070 (for a desktop) in about 1 year, they would go well together in my opinion.

The new CPU would have 35 watts more power that had to be supplied by the power supply and also cooled.

But then you would have to get an outdated board with an outdated chipset. Or is the board from the laptop also ITX / ATX standardized and can it continue to be used?

As I said, I see no use in it. Of course, you'll be faster, but that's single-digit percentages in real scenarios. Do you just have to ask yourself if you want to spend sum X on it. In most cases, games don't benefit from hyperthreading anyway.

The laptop has very good cooling, was also sold with an i9 9900k, and two power supplies, 2x 160 watts, should be enough. At most you can also buy larger ones.

You can do that, in the laptop the CPU will hardly bring you any advantages, in the desktop area the CPU is already outdated. I wouldn't get it and prefer to use a Ryzen, then you can also use PCIe 4.0