Then what?
Do you care to elaborate?
Someone unfollows you. Then what?
Probably questioning the utility of the tool.
Are you going to write that person and ask why? Are you going to change your tweets because someone doesn't like your writing style, political views or personality?
It's a fun little project, but it's of little actual value.
If they don't follow back after x days you can unfollow them again so that your 'Following' number isn't too inflated, and the ratio of 'Following'/'Followers' is better.
In the real world, 10,000 useless bot or "follow back" followers should be less valuable than a thousand interested followers who are likely to buy something and promote your business.
In social media world its easier for startups to get the thousand "good" followers if they also have 10,000 worthless followers because the good followers will dismiss a business if its only followers are the co-founders and their moms.