The problem I have with Stadia is that you need a monthly subscription and also need to pay full price for all games. Games that can't be sold on the second hand market, unlike the current console games. And at $129 it's not far off from the €200 I paid for my PS4 during Black Friday 2017. The TCO of the Stadia seems much higher than traditional consoles.
The upside is using it on a mobile device but to me this doesn't add value, as the network connection outside my house is either a high-latency 3G connection or someone else's unreliable WiFi. And with Remote Play on the PS4 I have similar capabilities.