There's no problem to solve. Your phone is doing as it was designed to do, which is to disengage if the devices are idle. It's not an opinion. It's a fact. Keep in mind Hotspot is a service from your carrier. It's more likely your carrier has mandated the Hotspot to disconnect if the connected devices are idle.
Whether you call it a threat or not, what are we supposed to surmise, when you ask if you need to buy a cheap Android as an alternative? That's a knee jerk reaction and it's one that's fine if you think an Android would be a better option for you. It just wasn't a necessary thing to say. All you needed to ask was why does the Hotspot disconnect if the devices are ide. And I explained why.
And I don't need to know how anything works on Android. This is an iPhone forum and I do know how things work on iPhone. Comparing two different platforms and expecting the same functionality isn't realistic. Android does what it does. Apple does what it does.
If you want to tell Apple you want them to adopt a different way, you can let them know your thoughts here --> Feedback - iPhone - Apple