They removed the device limit not long ago, so no worries there.
Whatever you do with them. The connection remains as long as you have it. If you keep the devices you can connect to internet and have them earn you BAT by viewing ads. I use my old Android that way. It has no phone service but connected to wifi and all is well.
But yes, otherwise, it just will sit connected on the backend unless you request it gets unlinked. It doesn’t hurt you since no limits.
NOTE
Make sure you have a sync chain for your bookmarks, passwords, etc and you get your new devices connected to it. Otherwise you may lose those things.