Selling your original system for $200 when getting the mid-gen upgrade isn't a deal. It's economics. If Stadia offers up yearly subscriptions for a lowered price, I'll factor that in when they do it. I'll also factor in lowered console prices, whenever the PS5/XB2 get their first price drop, instead of speculating on potential future price drops. We shouldn't be factoring in any price drops for either side until they are announced. That's only fair. Refusing to accept someone else's epistemology =/= moving the goal posts. You just ignored the extra internet cost for Stadia. I realize that not everyone will need to pay extra for internet, but not everyone needs to pay for XBL/PS+ either. So we can either keep the extra internet cost for Stadia, and keep the XBL/PS+ cost, or drop both of them. Either way the price of the consoles come out on top. You also ignored the cost of losing games you paid for, as well as the inability to resell them. And let's be honest, losing games you paid for is inevitable, when it comes to a streaming service. There's no speculation there.
"We shouldn't be factoring in any price drops for either side until they are announced." - Yet assuming $400 for a nextgen system is generous, this price isn't announced. You also assume $400 for a Pro system down the line.
"You just ignored the extra internet cost for Stadia. I realize that not everyone will need to pay extra for internet" - Exactly, my internet is already more than good enough and the cost has nothing to do with Cloud Gaming.
"But not everyone needs to pay for XBL/PS+ either." - Not everyone needs to pay for 4k streaming. After all you may prefer Ultra Settings at 1080p than Medium settings at 4k.
At the end of the day there are many different situations and options, either console or Stadia can come out on top.