This actually sounds like a good idea.
I don't see why adding one extra option to how long
you want the thread to stay tracked for should be too hard.
If you want to get everyone else's opinion, you should add a poll!

Edit:
Maybe instead of making it
automatically stay tracked forever unless changed,
there could just be another option added to your time range of how long you want to track it for.
Example:
1 day
3 days
7 days
14 days
30 days
ForeverVouch i think forever would be nice seeing as i keep my guild chat topics tracked and i have add 30 days tothem all the time and ive been tracking them for months so i think its a good idea.