100% is not a silver bullet and it is not going to give you 100% assurance that your code and your application is flawless. If you aim for 100% coverage, you may miss some of the important business cases (remember write test from the user’s perspective) which may give you diminishing returns. Instead, it is better to aim for 80-90% coverage and focus on the business cases instead.
Only aim for 100% coverage, if you write a library that is going to be used by the rest of the business.