I'd love to know why you think a higher priced console, finding many of it's exclusives going multiplatform next gen, and even with your own possibility of the Xbox finding some success in Japan, will somehow mean that the PS5 can only lose 1 Million units after 6 years on the market.
Everything is consolidating around single platforms. No more 3DS, no more Vita.
Just like how Nintendo had to consolidate their home console and handheld lines because they were stretched thin on resources for developing games for two separate platforms. The same occurred for Sony and third parties on PS4 and Vita. Japanese third party developers will now only have one Sony platform to develop for, this means the PS5 will have a stronger catalogue of games on one platform compared to its predecessor.
Meanwhile, even though multiplatform development has become easier especially for porting games, the Switch is still lacking AAA titles from Japanese third parties. I don't see the situation changing anytime soon when things didn't change in the last three years despite Switch succeeding.
By the way, I'm not predicting that the PS5 will sell 8 million units lifetime. Maybe it will do better than the PS4. Things are too uncertain when we don't even have launch details.
I just think that predicting Vita lifetime sales numbers looks at only everything that can go wrong with the PS5 which doesn't make sense when PS5 is expected to have a stronger start with third party support compared to how the transition went from PS3 to PS4.
There's no separate Handheld platform to take sales away from the PS5, the PS4 may still get cross-gen support for a while but the kind of people buying a PS4 this late in the cycle wouldn't have been deciding between a PS4 and PS5 in the first place. You buy the PS5 if you're expecting to want to play new games over time and not have new titles slow down to a handful after two years.
There, I have given you my opinion on why I don't think the PS5 will sell like the Vita in lifetime sales unless something catastrophic happens at PS5's launch.