The site is so biased, that a lot of people expect the PS5 to sell less than the PS4, even if Sony themselves said they expect PS5 shipments to surpass PS4 shipments (launch aligned) in 2023.
And Sony themselves killed the PS4, they are the only ones to made it collapse.
I'm sure many of them have had a look at the statistics and see that the chances that the PS5 outselling the PS4 would break some established trends.
Playstation + Xbox sales taken together have always amounted to ~175M each gen:
- PS2+Xbox: 183M
- PS3+360: 173M
- PS4+XBO: 168M
Meaning if the Xbox is stronger, then the Playstation sells less as a result. Since the XS is selling much better than it's direct predecessor, unless this gen breaks with the model it established the 3 previous generations, PS5 should sell less than the PS4 did.
That being said, there are some unknowns that could alter this a bit: How long this generation will be can have an effect for instance, if there will be some mid-gen upgrades like the PS4 Pro last gen, and so on. So far, noting shows that this gen will deviate from the previous generations, but time will tell if this will stay the same.
Another thing to note is that the Playstation + Xbox total has been dropping a bit each generation. This trend will have to be reversed for there to be any chance for the PS5 to outsell the PS4 since the XS will almost certainly outsell the XBO.
These trends are caused by the quality perception of their consoles. Xbox wasn't nearly as good as the PS2, so most gamers didn't think they were missing out on much. PS3 and X360 were comparable but neither of them were on the level of PS2 or PS1, PS3 had a terrible start and X360 a slow start. Xbox One wasn't as good as the PS4 which is universally considered better than the PS3. PS5 and Xbox Series should break this trend because for the first time, BOTH of them are highly desirable so far. Furthermore, Game Pass is a unique selling point that may appeal to a demographic outside the traditional gaming community, and also increase the percentage of multi console owners. VR has some potential too.
"So far, noting shows that this gen will deviate from the previous generations"
Demand for PS5 and Series consoles is much greater than PS4 and Xbox One. If this is considered "nothing", then what isn't?
If there is one concern to PS and/or Xbox eventually declining, it'd have to be PC. In the long run, PC may prove a bigger threat to console sales than either console is to the other, because it's virtually getting everything... games, accessibility, convenience, continued free online, and even portability. PC is just too appealing of a platform that you don't really need trends to recognize its potential to steal console gamers in very large numbers. It's kind of been happening for a while actually, I know a lot of console gamers who switched to PC ever since the start of the last generation.