There'd also be another huge problem: our site would be incredibly slow. Each file we load has its own overhead; when multiplied over many files, that overhead quickly adds up and leads to lost customers and lost revenue. Our customers expect the site to load as quickly as possible and it should be reliable!
There are also many ways to optimize our development environment. We can use testing tools like Jasmine and Karma to run tests. We can use a code linter to check for errors as we work. And we can also use a live development server that reloads every time we change our code.