https://theathletic.com/111725/2017/09/27/mirtle-is-the-nhls-shift-away-from-veterans-to-the-kids-a-good-thing/
Amidst the salary cap, free agency, and rookie count conversation, Mirtle touches on the biometrics the Leafs are using during practice and what the data might be telling them about peak performance curves.
We saw last season Nylander getting pulled from an upcoming game due to some mysterious ailment after a full practice. The theory at the time was something showed up weird in the monitors and they wanted to play it safe.
This point, in conjunction with Greening's remark that all the UFAs signed by the Leafs for the NHL roster are all upper 30s, but still in the league and considered effective piqued a theory.
Originally, when they were signed, my take was that they wanted playoff veterans who had the right playstyles to match the young roster; players who also had the 'intangibles' of living right, gyming hard, and eating properly. Having them in the room with the younger players to impart a lot of that through osmosis and mentorship, teaching them how to have longevity in a league that chews the majority of its players up, and in the case of Marleau how to handle being a highly touted star and staying grounded. They're also at the stage in their careers where they are right on the cusp between still-effective and overtake-able (hence the short contracts).
Now I believe there is an additional benefit: science! The Sports Science & Performance department is gathering oodles of biometric data on every player in the system, the elites, the primes, the prospects. Now they have a small sample of Methuselahs and can start finding data markers for what sets Marleau, Hainsey, and Moore apart from the rest of their cohorts that have since retired. It's not just right living and maintenance that gets some players up to their 40s playing a high speed contact sport effectively. The Leafs have a lot of technology at their disposal and now they also have some guinea pigs to really test their theories and techniques on for refining the development program and finding advantages in scouting.