Your knowledge is incomplete on several points. How do you know how many adults play Switch? Because there are many. How do you know how many people play Switch in TV mode? Because there are many.
Price matters a lot in a world where the next generation of consoles has to target a much higher selling price than before. A technically advanced desktop console with up-to-date graphics can now be purchased from a starting price of only $700 due to the rising prices seen in the industry. It makes no sense to come up with an expensive basic console at such a price, as evidenced by the market's ability to absorb it based on previous similar examples. Parts prices are no longer falling, and the proof of this is that none of the console manufacturers have reduced the prices of their products that have been out for 4 years. Another important aspect is that the nextgen handheld console will not only be used in mobile mode, but can also be connected to a TV, so it can also be used as a traditional desktop console. Although it will not be as powerful as a high-end PC, that is not its purpose. Its purpose and function is to provide entry-level console entertainment at an affordable price. Then a secondary upgrade hardware can be added to this, which raises the gaming experience to a high-end level. For those who want more and are willing to pay for this experience. This model has much less risk than an expensive main unit console. The point here is to reach customers, which can be achieved much more efficiently with cheaper entry-level hardware, and according to the current state of the technology, this can probably only be done with a handheld.