How do you define mainstream gaming? I still don't think these cards are mainstream, like at all.
In Steam hardware survey, the 50th percentile gaming PC is playing at 1080p with <8GB VRAM and 16GB of RAM (with most of the other half at 32GB, which was a "massive swing").
It's disappointing that PC gaming has been stuck at 1080p for like a decade, but it is what it is.
From a typical user perspective, these 1440p60+ ultra cards are targeting a market segment that is higher than mainstream.
Another way of looking at what is mainstream is by what is popular:
The top 10 GPUs are definitely getting more expensive lately, especially since Nvidia stopped halted manufacture of <$300 gaming cards - but only 2/10 cross the $500 barrier and you're still looking at an average MSRP of around $400-$450 depending on which 3060 version and how you price a laptop GPU.
February is an outlier though.
January was closer to typical though, with only 1 >$500 MSRP card in the top 10 and an average of around $360.
A third way to look at it is to just say "Nvidia controls the market and is smarter than us, so mainstream gaming is whatever they theoretically say their xx60 cards are worth" Which right now is either $300, or nothing. Because there is no RTX 5060, and RTX 4060 production has probably been stopped in favor of higher margin cards.
Steam Surveys are not the best metric, and applying USA MSRP pricing to a global snapshot of usage is not so good either - but I'm not finding any way of looking at it where its justified to start calling $550 "Mainstream".
I don't think it's a good idea to base what we call mainstream on the currently offered product stack though, because people do not have to buy a GPU to play games. If Nvidia stopped making every card except $2000 5090s, that would not suddenly turn the RTX 5090 into an entry level card. PC gaming is not an essential, and other forms of entertainment are easily available everywhere, for free. When people are faced with the choice of a $500+ gaming GPU and nothing... most of them are going to choose nothing... Or, they choose a PlayStation.
I think a debate for what counts as mainstream gaming is *today* should start somewhere in the ballpark of a 1080p60 ultra card that is somewhere in the price range of $300-$500.
The RX 9070 XT and RX 9070 just don't meet that definition.
Of course, all this all is just limiting the topic to dedicated PC gaming and ignoring true mainstream gaming, because 99%+ of all games are just played on people's phones.