Serious impact of tight coupling? without using Dependency Injection

For the developers who started off programming during early 2000's might have seen lot of code which were very tightly coupled. I'm a software-dev since 2016 and from the beginning I've never seen code very badly written. They were very loosely coupled.

1. So, I fail to understand the serious consequences what would happen if the code is tightly coupled.

2. What amount of time (in days) will take to extend a new requirement had the code been tightly coupled?

I know that this topic is discussed a lot earlier but for me,

• I'm failing to understand what is the serious impact of not following loose-coupling principle.

• I read through Dependency Injection in java and I get the intent of why it is there. But I'm failing to understand the SERIOUS impact if it is not used.