You can have a trillion dollar networking system and still stream sh*t quality content. All streaming services have their own QC for streaming. Does Roku support 8K? Yes. But at a cost in quality for streaming. Roku has I think a max bitrate of 40Mbps/VBR (Content). That's low grade, a light load, but very streamable.
I'm paying for 500Mbps. Cable Modem DOCSIS 3.0/800Mbps/1Gbps Wired. Wireless Router has 480Mbps/2.4Ghz and 1.3Gbps/5Ghz. My PC is connected by WiFi Adapter 5Ghz and showing 975Mbps-1.1Gbps. That's in-house speed/Local. Am I going to pull all 500Mbps from my service provider? No. On a good day I may reach 200+Mbps. Is my cable modem/wireless router old, yes.
PC. I can stream 4 1080p/24 without lag across my four monitors. But their are wtf days.
Also, I don't have a lot of devices on my network. Mentioned before, my TV has dual bands, but never pushed passed 50Mbps. They said it was hardware limitation. So no reason to have it on 5Ghz.
https://www.cablelabs.com/technologies/docsis-4-0-technology
https://developer.roku.com/docs/specs/specs-overview.md
https://www.roku.com/blog/8k-roku-tv?srsltid=AfmBOoqoBxodcdAgNyt5FvsaTKyaZfPdM4zTATmWvsHQjIuPX3XblD24
Also, Roku is software, not hardware. Their not trying to be a dedicated AVR or trying to be NVIDIA or Intel, AMD etc. Could they? Sure. Are there other wireless brands out there, better then Roku? Yes.
If you look where technology is and headed, WiFi6,7 are slowly becoming outdated.
Roku is like fastfood.