For countries like UK & US where we have privatised (incredibly expensive and often relatively unreliable or low rural coverage) broadband access, as well as a well established, many decades old home console/PC market, yeah even if Stadia was perfect on Google's end tomorrow it'd take many many many years to catch on. But for many countries, much of mainland Europe, where you can get ~gigabit FTTP/FTTH for the price of a basic package here, or over the next couple years when there is 5G with lower latency than our copper-last-mile home broadband, combined with the fact in many of these countries physical games & hardware can seem quite overpriced compared to the UK due to the smaller markets, things like Stadia could be a massive hit within a couple of years, now the internet infrastructure is there.
In theory, developers could eventually be making games that are literally impractical to attempt to run on a home PC, if games are made for cloud hardware from their inception they could have the potential to eventually start levaraging much much larger pools of fast local memory or many core systems long before devs could realistically target these for home PCs. Combine that with 1ms internet and 120fps targets, yearly hardware upgrade cycles to match the needs of new games and so on and in 5 years time these services could technically be the "top-end" gaming systems.
