I initially thought this would be problematic for my parents since their iPad is currently used as a hub to control some light strips. They do not want or care to have an Apple TV or HomePod. However, after doing some testing on a spare device using the new Home architecture and no hub, I think they actually don't need a hub for their use case and wouldn't be affected much by the upgrade.
Their automations are currently set up in the Home app (all time-based automations), and require a hub as a result. During my testing, it looks like their automations could be set up as personal automations in the Shortcuts app instead. They would still be able to use Siri on their iPhone or iPad to control the lights as well.
They would lose the ability to control the lights remotely (when not at home) or use some of the more complex automations (i.e. last person to leave home, a sensor detects something, or an accessory is controlled, etc.), but they never used those anyway. They would also lose the "only run when I am home" capability, but they take their iPad with them when they travel, so that's not really a problem either since the iPad wouldn't be home to control things anyway.
You probably don't need a hub if:
- You only have time-based automations controlling a few devices. Look into setting those automations up in the Shortcuts app instead.
- You have no automations and don't control your devices when away from home. You can still control them from your device or Siri when you are home.
You will need a hub if:
- You have more complex automations (i.e. run when first person arrives, run when last person leaves, run only when home, etc.).
- You have devices that control other devices (i.e. a switch or motion sensor that controls a light).
- You want to control your devices when away from home.