The console business model has always been to sell the platform at or near loss and subsidize that with software sales. Introducing third party just means the Xbox consoles won't look cheap compared competitors.
If anything they'll confuse people into thinking they are PCs for your TV. Certainly you'll lose some edge from exotic design when you have to coordinate more manufacturing specs.
Xbox subscriptions are falling and will likely see a price rise. So they are taking a big gamble with this.
Forgetting for the moment that (for a few generations now) 'PlayStation' & 'Xbox' are merely stripped down 'PCs' with custom APU's and Operating systems.
While in days-gone-by the intention was to 'lock' owner/users into your 'ecosystem' with exclusive titles.
This later morphed into (the holy grail of business!) of reoccurring revenue (IE: monthly subscriptions).
As PC OEM's the world over take chances and risks developing a multitude of 'PC' form factors and bringing them to market, what percentage of these end up being subscribed to 'Xbox' services?
So team MS is thinking that greater 'Xbox' hardware choice is needed here?
As the well-known journalist Jez Corden has confirmed, the new Xbox could serve as a reference device and offer other manufacturers the opportunity to launch their own Xbox devices on the market.