the app's minimum android version is actually depend on how or what the dev want the apps to do.

Usually newer android has better library that the dev can use, for example; popup. You can make this feature from scratch and support as many android version as you want, but this might give you a nightmare.

I remember wanted to be a "good dev" and want to support as many version as possible, but end up supporting latest android only because feature that i wanted to use wasnt available in the older android.

To answer your question: If the apps required newer version, and your gadget no longer supported, then yes you need to buy newer gadget.