Xbox is offering a no upfront cost payment plan with the XB1 and Scarrlett. If Sony follows suit, Stadia users will largely be using the free version. There might be a blue ocean of people out there with a good internet connection, that don't care about resolution, and don't want to pay a monthly bill or buy a console. But those people were never going to get a console or PC anyway. So it's not competition with PS5/Scarrlett at that point. Kind of like how the Wii wasn't really competition for 360/PS3 back in the day. So if Stadia has any amount of success at all it would be pure growth for the industry. That's a good thing.
I could see Stadia getting 30 - 50 million people using the free version by the end of next gen. That's a pretty low number considering it's a free service. One person buying five game on the free version of Stadia =/= one person buying a console and fifteen games.
Microsoft tried to do the financing thing when the 360 with Kinect came out. I worked at Best Buy and I think we sold like... 3 of them. TBH if anyone really was interested in that when we mentioned it, we'd push the Best Buy credit card instead. Which to be fair wound up being cheaper for the customer anyway. Might be more attractive with game pass, but it still winds up being 100 dollars more in the end, while most store cards would give you interest free financing for 1.5-2 years anyway.