Hopefully single player only games should support that. It's a standard supported configuration on xbox one right now to connect any USB3.0 >250GB HDD externally and move games onto that.
With PS5 there should be a number of single player games where greatly-increased loading times won't have a negative impact on anyone else, unlike multiplayer games where everyone should load into a map at the same time. I have in mind the sequel to Horizon Zero Dawn, and the upcoming Assassin's Creed.
The push for these crazy fast SSD’s when even a SATA3 SSD would be a huge leap for load times isn’t just to reduce load times but to allow on-demand streaming of assets just in time. An external HDD isn’t going to cut it for the random reads developers looking to get the most out of the hardware are going to be doing.
Archiving games you aren’t currently playing because a USB hard drive is most certainly the faster option for anyone without a gigabit internet connection? Sure. But there’s no chance in hell they will let you run games off one.
I wouldn't be surprised if they shuffle games between external drives and the SSD _almost_ on demand. The HDD might not give the random access and burst speeds new games need, but if they can maintain 100MBps they can transfer a 100GB game in about 15 minutes (and faster if the game can prioritise the assets it needs or boot before it's fully installed). That's not the end of the world - I've had longer patches on PS4.
With PS5 there should be a number of single player games where greatly-increased loading times won't have a negative impact on anyone else, unlike multiplayer games where everyone should load into a map at the same time. I have in mind the sequel to Horizon Zero Dawn, and the upcoming Assassin's Creed.