In my experience you should should never buy a product for what it might be one day and instead only pay for the what you're getting on day one. That way you're never disappointed/always get what you paid for.
For example I recently purchased an Oculus Rift. I waited until enough games released so that I couldn't be disappointed by the experience/value, and I haven't been. Anything else added after this is just value add.
> I want to play Red Dead Redemption 2, the cheapest way to do that as I don't have a console nor an expensive computer is Stadia.
Stadia costs $129 with no games. An XBox One S (Digital) on Black Friday, including a game, costs $200 or $150 with no game (e.g. NewEgg). So while your point is accurate, there isn't as much in "cost" as you'd think, and I'd argue that you get far more value with an XBox One S than a Stadia (even the digital one that cannot play BluRays).