I think you're bang on as far as a bell curve distribution of coach impact.
But I don't think that reflects coaching being overrated so much as many individual coaches get overrated, or underrated, because it's difficult for most fans (& a surprisingly large number of GMs/Owners) to fully understand the complex nature of variables that ultimately drive results in high performance sports.
Not sure it's fair to critique MSL as "relying on vets". Of the ten highest toi/g for the Habs this year, only 3 are over 25 (including recently added Carrier), only 1 is over 30, and only 3 have played 250+ NHL games (Suzuki, Matheson & Savard).
Last year was the same as far as players over 25 (swap Carrier/Monahan) & the year before that as well.
Reality is that we've had a very young roster since the rebuild started and the coaching staff has, if anything, shown an impressive commitment to live with the growing pains of youth. A typical coach would've been riding Anderson, Gally, Armia & Dvorak far more heavily if the vet bias were applicable here imo.