Not sure what you're talking about. Keylor will be 32 in December, for a goalkeeper living primarily off reflexes on the line that's the twilight of their heyday usually. I'm a big fan of him but it would be absolutely wrong not to get a great gk in that situation seeing how other clubs are paying 70-80 millions for unproven ones. For half the money that Chelsea or Liverpool spent on one keeper, we have the goalkeeper pipeline filled for the next 10-15 years. From a financial standpoint the Courtois transfer was one of the best buys in the entire summer transfer market as we got him under par value and there's always the opportunity to sell players again (although I don't think it will happen in this case because of the age of the players involved).
And as much as I like Navas he won't get any better. It would be foolish to play everything on one card and pray that he stays healthy, especially after he already had surgery on his achilles tendon about two years ago and seeing how he had his mistakes (e.g. Juventus return leg last season, first goal in the European supercup this season). I would rather have two great options than only one, especially after seeing how Casilla played last season.