Crytek has financial worries. Plus it's the most difficult to use, because it's almost all C++.
Unity is very accessible thanks to C# scripting, but has bug issues (and no source code).
Unreal Engine looks amazing, is cheap or free for indie studios, and has source code included.
If they can make UE slightly easier to develop for, they'll dominate the market.
I think Unity is too valuable to low-medium end indies to go anywhere (you really don't need most of these new features if you aren't going for a photorealistic look anyway), but we could see a partitioning of the market as Unity takes over the low-mid and Unreal takes over the high-end. Unity actually no longer has the (partially) realtime global illumination they were so proud of just a couple years ago, because of licensing woes: https://blogs.unity3d.com/2019/07/03/enlighten-will-be-repla...
Plenty of proprietary engines are in use today. And you've got FOSS options like Godot and Blender. UE's definitely the biggest, but other options aren't going away any time soon.
Unity source code is partially available on github, and full available on commercial license, which is fair.
Then there are plenty of other engines out there, for example Cocos2d-x is number one in Asia for 2D mobile games.
Both engines have bugs, and one of the biggest complaints about Unreal Engine is that any feature not used by Fortnite is half-baked until Fortnite uses it. With Unity, the biggest complaints are about the relatively transitory lifetimes of some of their biggest-marketed features, like rendering pipelines or networking code.
And then there is always the possibility to pay for the source code anyway.
This is a business after all.
More recently it's been making a lot of noise about its new scriptable render pipeline and shader graph. Of course these are still playing catch-up with Unreal, but they're doing an admirable job.
Meanwhile Unreal totally changed its pricing model ~5 years ago to be much more similar to Unity's and affordable for indies/individuals, as well as adding native support for mobile platforms, etc.
For some customers there may be a clear choice, but for now at least there's also a ton of overlap between their markets. We'll see where things go in the future.