There will be no "nextbox" or "PS6". In 0 years time, Xbox and Playstation will be storefronts/platforms with streaming apps that you will launch on your smartTV. The economics of it makes this scenario inescapable: what is going to go down in price more quickly (if at all)? Bandwidth and dataplans or silicon?
No, there will
definitely be a PS6 and Xbox Next in seven or so years' time. Streaming, even the best setups like Stadia and Xcloud (or even with the Origin tech I hear about from iD time to time), still have latency that impacts certain levels of precision performance in competitive games. Now it can be argued that since those playing such competitive games are just a sliver of the total console market, if left with no boxes to purchase they would simply go over to PC.
Which is almost 100% true. However, I think the ongoing issues with Apple, Google etc. and the fight for storefronts to exist as genuine storefronts on their devices shows that technological limitations or advances won't be the only (or even major) deciding factor in enabling or (IMHO) defeating an all-streaming 10th-gen. It's going to come from the fear of fallout that results from loss of vertical integration. Microsoft already is feeling this with getting GamePass and Xcloud on Apple devices; if Sony were to try something similar with PS Now, they'd be met with the same pushback. There is a level of security afforded by fully controlling the software and hardware stack, once you give that up it's nigh impossible to get back.
At most, supposing 10th-gen is streaming-based, the very least we're still going to get Sony and Microsoft PlayStation/Xbox-branded streaming box clients that they will use to provide their cloud streaming services through with full vertical integration. Out of the two, Sony would be more reluctant in providing that streaming option on at least non-Sony devices, but they could technically extend its offering beyond a PlayStation device to their phones, televisions, etc. For Microsoft a cloud streaming-based 10th-gen Xbox would be something more of an option, since they are more receptive to providing GamePass and Xcloud wherever it's accepted, including other hardware ecosystems (Apple devices, Google devices, Samsung devices etc.).
There's...actually perhaps an upside to a cloud-based 10th-gen system; the local hardware power needed for such a device would be much less than building millions of consoles needing to all provide that gaming performance through local native hardware. Saving on BOMs by hundreds per unit, putting that budget onto other aspects of the design to ensure streaming capability is top-notch. They would still develop physical 10th-gen system designs at the hardware level of course, but they would only be for their server clouds. Upside here is that they could design more powerful/capable designs since the volume of instances they'd need to manufacture for their server blades would be magnitudes less than producing 10s of millions to 100s of millions of the units over a console lifetime.
So then that budget excess could be moved towards other hardware; 10th-gen I think, Sony will want to try standardizing a VR/AR experience while still complementing a traditional experience. So the money they'd have to put towards GPU, RAM, mass storage etc., could be shifted towards inclusion of some next-gen VR headset, with wireless as a standard option, pushing VR further with more innovations etc. I don't know what that does for the PlayStation gamers who'd just want a "traditional" PS6 though; not just those who have it as a preference but also those who require it because they don't have good internet for streaming. Would Sony just let them move to PC? Possibly, but maybe that's where the shifting strategy of bringing more PS games to PC comes in. I think we're going to see a lot more advancement between Sony and EGS, I won't be surprised if eventually Sony and Epic enter some mutual profit model and EGS is rebranded to something PlayStation-related, folding PS Now into it as well as PS +, online multiplayer becoming free and that rebranded service/storefront also becoming a part of PS6.
Especially if Microsoft do by some chance end up purchasing Valve/Steam, I could definitely see this happen with Sony/EGS going forward. And maybe Sony just basically soft-rebuilds their PC division under a PlayStation branding and try their own Steambox-type model with their own OS, integrating the OS and some PS/EGS storefront service in one integrated package, throw a PS controller in there and price it aggressively/semi-aggressively (vaguely similar to what a console'd cost, but somewhat pricier) to attract the PlayStation people who still want a native/non-streaming gaming experience without completely sacrificing them to open PC.
Microsoft, like I was saying before, they'd be a lot more content in this timeline to just put out a streaming box for 10th-gen; any Xbox people who want a native experience just go to PC, this works out particularly well if MS somehow acquires Steam. I doubt they'd do a Steambox-type approach, though in such a case or, if they did, it'd be at very small numbers. But again, this is ALL assuming you're right about an all-cloud based 10th-gen which, I don't necessarily see being the case. But then again, seven years is a long time from now, and anything can happen, so I definitely keep this option open