Of course getting stronger faster leads to diminishing returns later on. If you improve quickly at anything--maximal strength, speed, flexibility, mobility, sporting technique, etc.--you're reaching your "ceiling" faster, which means you'll improve less in the future. There are ALWAYS going to be diminishing gains: neophytes will always improve the most in their first year, and in each subsequent year, the gains will be less and less.
Think about how ridiculous this sounds: you'd never tell someone to hold themselves back in learning technique for a martial art because of "diminishing gains," would you? Of course not. You want to improve everywhere as quickly as is reasonable, without detracting from your training as a whole. If you're a fighter who can get much stronger over the course of a year without detracting from his or her overall training, then it makes no sense to hold yourself back and remain a novice. Sure, you're leaving yourself more room to improve for the future, just like I'm leaving myself more room to improve my cardio for the future by, you no, never doing cardio.