You would be surprised how fast things are improving.
If I was a betting man... Uncharted 1,2 and 3 will likely be fully playable in the next 12 months on even slow CPU's like the Ryzen 3 2200G, it already is on higher end systems.

The WiiU, Xbox 360 and Playstation 3 CPU's, whilst they might all be PowerPC derived... Are all pretty different from each other.

The WiiU's CPU though has allot more modern features, which is why that chip can translate so well.

Simple is all relative. The SPE's on the Cell are Simple. The Cell as a whole is actually fairly complex... But emulating chunks of a chip like the SPE's is actually simple.

Indeed.

Xenos is a hybrid chip that pretty much bridges the R580 and R600 architectures, much of what is in that GPU is most certainly a known quantity at this point.

With that said, the Cell has certainly not been a hindrance in emulation efforts like many people have claimed in the past on this forum.

I really think the safe bet is 9th gen consoles will have BC for 8th gen. I would hope MS's effort of OG Xbox and 360 emulation come to the next Xbox.

As far as PS1-PS3, I agree that will have some sort of paywall but maybe there will be emulation so it doesn't rely on streaming. I suspect a Game Pass clone is coming.

I don't think the emulator will ever get so efficient that a 2200G will give performance at par with the native hardware in the most technically demanding games. But I do think it will eventually get to a point where a 2200G makes games playable if optimization continues to improve like the Cemu emulator. Which is good news, Sony could launch the PS3 Mini in a few years with relatively inexpensive specs.

It already does. In in-game cutscenes it will do 60fps in the uncharted games.
It's when it jumps to gameplay that things take a nose dive, but the emulator is still very much work-in-progress and improvements are being made.

Allot of the better supported games run silky smooth even on the 2200G.

Well the Cemu is much more efficient but still struggles to give native performance on a 2200G.

Whatever, I hope all of these emulators get to a point where that would be enough to run at least as good as native hardware.

How much time do you think will take for us to see an emulator of PS4 running UC4 in 60fps on SP and perhaps even adding 4k textures?

From the Cemu website: "Cemu is not intended for general use yet."
Just because it has an edge over RPCS3, doesn't mean it is ready and as efficient as can be.

It also doesn't support Vulkan or Direct X 12... Which tends to be the targeted API's that are being optimized most for AMD and nVidia's hardware.

With that said... CEMU can do Breath of the Wild at a locked 30fps on the 2200G, nothing to sneeze at.

Hard to say... The rapid development of RPSC3 has blown me away at every turn... The Playstation 4 inheriting allot of it's "tricks" from the PC could actually aid emulation efforts.

All of the efforts towards emulation of modern systems we've seen has been thanks to HLE (high level emulation) design ...  

Older systems lacked the concept of a kernel which was an abstraction layer between the software and the hardware so emulation of older systems involved using an LLE approach which was far more computationally intensive since the host system had to imitate the bare metal itself. The beauty of Cemu and RPCS3 is that they made a conjecture of what if instead the games were actually running on top of the OS in the background at all times ? 

As more recent systems adopt an ABI and a gfx API, this will make them inevitably more vulnerable to emulation since those components can be reimplemented at a much lower performance cost. It's the price both platform holders and developers are willing to pay in the name of productivity as they rely more on code gen from the compilers ... 

A higher level does not have to necessarily lead to lower performance as long as the abstraction matches the hardware. The biggest problem with Vulkan despite it's lower level of abstraction I think is that it's not a great abstraction for the modern AMD/Intel/Nvidia hardware which means PS4 or Switch emulation won't get very far with Vulkan since the gfx APIs biggest design flaw is that it doesn't support bindless ... (emulating those systems with Vulkan will prove to be very problematic since they support pointers in shaders) 

Not really disputing any of that.
Sometimes Vulkan/Direct X 12 only brings marginal performance improvements to emulation anyway.

However... That isn't the aspect I was looking at... It's driver optimization from nVidia and AMD... And Vulkan/Direct X 12 is where the bulk of their attention is currently.