That may be the case, but how low are they willing to go with the price if they originally expected to ship THAT many? Especially when the Series S is going for as low as $299? It may not mean much in Europe and Asia, where PlayStation dominates, (Well, if you don't count Nintendo anyways) but in North America, where Xbox can actually, seriously compete, that could really tip the scales in Microsoft's favor.
As for your first point: Yes, the PS4 stock was limited during the first 6 months of its lifespan, but I doubt that demand for the system in its early stages was as high as 11-15 million units shipped through March 2014. 11-15 million in it's first 4 1/2 months?!? That would be at level the Switch is selling now. There's no way demand was that high. And I seriously doubt that it's that high now, especially when the PS5 will be up against more competition than the PS4 was at the time. Microsoft, specifically the Xbox division, is in a FAR better place now than they were in late 2013, both from a PR and pre-launch standpoint. After that Xbox One reveal disaster, they have nowhere to go but up, which is the direction they appear to be headed. I fully expect Xbox Series sales to be notably higher than the Xbox One was during its first few months. Those extra sales have to come from somewhere. Plus, there's also the Switch, which is just on an absolute 'nother level. (Especially compared to the Wii + 3DS.)
I just think that Sony has more going against them this time around than they did during the PS4's pre-launch phase, where I think they're setting themselves up for disappointment if they expect actual demand to be as high as they're projecting.
Sony shipped 7.5M PS4s by March 31 2014, as you are aware. This article suggests that they were aiming to double that number to 15M. But now landing at 11M. So basically just 3.5M units more tan they shipped PS4s in the same time frame.
The switch doesn't affect sales of the P/Xbox consoles any more than they affect its own sales. So I don't think that's a real issue here.
NA represents only 35% of PS4 total sales, but almost 65% of the Xb1 total sales. Sony could very likely be planning on sending only like 4 of those 1M nits to NA, and the rest to everywhere else. Xbox doesn't have as strong a presence outside NA as sony, this could just be sony playing to the strengths they know they have.
Lastly... at launch (especially at launch) console sales are driven mostly by the core base of the platform. Basically, there is a certain number of people that will be willing to buy a PS5/XBSX/S as long as those units are available and it hits a certain price. I think Sony would be able to sell 11M consoles in its first 6 months on the market if stock is available and the price is right. And we don't even know that the PS4 couldn't have accomplished the same feat, or that the Switch couldn't have sold even more than it did, all we know is that they were all stock limited.