I think this more people mistaking people expressing their preferences for a system and extrapolating that to meaning market share predictions.
Reword the question to do you believe Steam Deck will overtake Nintendo market share and you'd get different answers. Same with if you ask someone why is Linux better than Windows versus do you believe Linux can overtake Windows market share?
I find people on the internet have a hard time differentiating between people who are expressing preferences and people predicting market share shifts. People just see oh this person doesn't like Nintendo or Windows and must believe Steam Deck or Linux is going to be more popular.
What I wrote might have been confusing, but I was trying say that places like lemmy may have view points that express preferences that aren't representative of the mainstream. Like how there may be more positive Linux comments on average per user.
But, that it doesn't necessarily mean the people expressing those views believe them to be representative of the mainstream. It is more just them expressing their thoughts.
However, people I found across social media can mistake what are simply individual opinions as general proclamations, and immediately jump to "Oh this person is claiming that their view point is one most people hold. What a bold claim." When all they were saying was I like turtles as opposed to most people like turtles.