Well there is nothing preventing Dex from coming with a monthly plan. Note that all plans are very low interest rate loans, there is nothing special about a phone plan.
Also, majority of people just use email and word suite. An S9 is easily capable of doing these tasks and it saves the individual or company cost of a cpu box (400 USD at the low end).
Plans are also just generally a good way to hide the actual cost of the device. Notice that, when you walk into a Verizon store, they aren't saying "BUY PHONE FOR $760!" they're saying "A New iPhone for just $32/mo!" So most of these people aren't going "Oh MAN only $760 in the end?!" they're going "I can afford $32/mo!"
My point is, it really doesn't matter what the reality of the cost/loan is, people who think "$30/mo! I can do that!" are probably not also going to think $200 seems like a good price for basically a cable to plug my phone into my computer.
Obviously, your other point was that it could come with a plan too but that seems doable but also there's yet another cost on top of the phone they were just barely justifying already. I'm not saying you're wrong but I think the price of this device is ridiculous.
I might pay $80 for it if it were really something I needed but, as others have said, I don't carry my phone around to be a computer. I carry it to be a phone. On top of that, I never really have a situation where I've got a keyboard, mouse, and monitor just sitting around waiting for something to activate them. So I'm not sure when I'd use this anyway.
Also, majority of people just use email and word suite. An S9 is easily capable of doing these tasks and it saves the individual or company cost of a cpu box (400 USD at the low end).