Doesn't PS5 already have a lower priced sku of it's own? Rather ironic you're speculating about things based on rumours of prices and an unannounced system, yet conveniently overlook something that's already confirmed.
Lockhart, whatever it is will still share the same architecture as the Series X, and still be considered a next-gen system. No different to Xbox One X or PS4 Pro from this gen in that respect.
Revenue made on console sale prices matters very little (see: Nintendo profit margins). What matters most is getting your system in people's homes. So yes, Xbox gaining more market share would be worse for Sony and good for Microsoft.
What is the PS5 lower priced SKU? Are you talking about the diskless PS5? That might be $50 to $100 less than the disc version, and it would still have the same specs as the main PS5. So I don't quite see that as a different system.
So why do specs suddenly matter now?
Both your title and OP completely phrased the issue around pricing, stating 'Sony doesn't have a $300 sku', and conveniently ignoring their digital-only system that's already been confirmed, and likely to be $100 cheaper.
So is the magic number $200 less than your competitor? Is that when it becomes unfair, or not equivalent anymore?
As far as we know, Series S or 'Lockhart' will play all the same games as Series X, with the exact same architecture. So outside of some performance sacrifices, it's the same system.
Regardless if it meets your requirements, as long as it helps sell more Xbox systems it'll be a gain for Microsoft.