The key thing there is that Declan Rice is 22. Even if he does turn out to be a good-but-not-great signing, you've still brought in someone who will be a regular starter for club and country for the next decade, which makes it easier to justify the fee over the long run.
You could make a comparison with someone like Jordan Henderson. A good but not great player Liverpool overpayed for but I doubt they perceive it that way nearly 400 appearances and several major trophies later. There's more than one way to ultimately justify a big fee.
And Rice is a hell of a lot more highly rated than Henderson was when Liverpool signed him. He could quite easily go on to be a lot better than "good".