a defect is a defect, new or existing. If we can keep all tests pass every day (in fact, multiple regression daily), there is no difference.

For real test automation engineers who master maintainable test design and real continuous testing, UI tests are not flaky, check out my other articles.

Automated tests <=> user story, so, that won't increase a lot. Just cover every user story with one or two UI tests, the team productivity will increase a lot.