One can't develop for what doesn't exist. Hardware isn't getting faster either. The PS4, Xbox One have slower CPU's than their predecessors, just with more cores (which makes backwards compatibility nearly impossible without recompile's.)
The Xbox 360 CPU was 3.2Ghz PPC with 3 cores, the Xbox one is an x86-64 with 8 cores at 1.75Ghz (X 2.13Ghz). The Xbox 360 has 48 shader (unified) cores, the Xbox One has 768 shader (X 2560) cores.
The PS3 has one 3.2Ghz PPC core with 7 DSP cores. The PS4 is an x86-64 has 8 cores at 1.6Ghz (Pro 2.13Ghz). The PS3has 32 shader(non-unified) cores, PS4 has 1152 shader cores (2304 Pro)
By comparison, The PS3 GPU is a GeForce 7800 GTX, while the Xbox 360 GPU is comparable to a AMD X1900. Their CPU cores are roughly equal to that of a "Core Duo" systems from the same era.
The PS4/Xbox One GPU is roughly equal to a AMD RX 480, While the Pro/X models are closer to a AMD RX580, at least in "power". The RX480/RX580 is basically direct competition to the Geforce GTX 1060. The CPU's are equivlent to about a AMD FX8120, and have no Intel equivalent unless you count hyperthreading (which cuts the ALU in half), by which then a quad-core Intel Core i7-4700HQ @ 2.40GHz is probably the closest thing.
Which means that the PS4 Pro/Xbox one X, is roughly on par with a PC that has a 2.4Ghz CPU with 8 hyper threaded cores and a Geforce 1060. Your average laptop falls very far short of this.
A developer can not target a PS5 without a devkit, and no devkits exist, instead the "Pro/X" models came out as 4K versions of their previous models, otherwise unchanged. 4K performance is going to be miserable on these devices as they can't do 4Kp60 without sacrificing everything else, so you're still better off running these at 1080p. Likewise many 4K Monitors and Televisions don't actually support "4Kp60" because they don't have HDMI2.0 ports, and HDMI2.0 only allows for 4:2:2 pixel encoding at 4Kp60, so you lose half the color information. Rec.2020 still isn't available yet because no panels actually have true HDR yet. A theoretical PS5 would have to target 8K, Rec.2020, and there just is no hardware that does that yet. To do a 8K render presently you would need at least two Geforce 1080Ti's, and most "gaming" PC's aren't capable of this as the PCIe Bandwidth doesn't exist on anything designed for desktop use.
No, I think we will see 8K's only real application in VR, and it will have a limited shelf-life, since people don't want to destroy their eyes. That is a dead end without some drastic change in brain-computer interfaces. HMD's simply will never take off as anything other than a way to privately watch video or play a game without distraction.
CPU's aren't getting any faster, they don't have any where else to go to shrink the die. GPU's are likewise tied to the same problem. So expect the PS4/XboxOne X to stick around longer than the PS3/Xbox360 by virtue of the fact that the CPU/GPU's won't be getting any faster, just fatter. Instead of trying to have 5Ghz CPU's, we will instead see 32 core CPU's that operate at 2.4Ghz. Instead of 3Ghz GPU's we will see 1.5Ghz GPU's with 8000 shader cores (eg two Geforce 1080Ti's glued together.)