The benefits definitely diminish, but not because you get used to the exercise – it’s because your fitness level gets closer to the optimal level. Your strength and endurance can’t increase indefinitely, for a variety of biological limits. Your muscles have a maximum size that is strongly affected by genetics, and the same is true of the strength of your tendons and the oxygen-carrying capacity of your lungs.
Sustained exercise at high levels (such as running more than 48 kilometres per week) has actually been shown to have a negative impact on your long-term health, causing permanent damage to the muscle fibres and nerves in the heart. A 2013 study of over 52,000 cross-country skiers found that those who had completed the most races had the highest chance of suffering heart rhythm problems.