Anecdotal counterpoint, the best teams I've been on have always had a good mix of a couple of really senior/decent intermediate people and a few either totally fresh grads or juniors (at the beginning of the project). Those fresh people have a good chance of becoming pretty formidable pretty quickly with the right mentoring, and without them seniors have a tendency to just remain experts on whatever tech stack they're familiar with but not think out of the box.

Hiring a mediocre senior is much worse than hiring a grad because they will never get any better, and it's very hard to know at hiring time that they're mediocre.