The problem remains depth though. Otherwise I agree.
We should base our primary game plan based on our starting 11 and adjust from there based on who is injured and who is available. The game plan shouldnt hinge on the availability of just one player.
In the Conte era, the 3-5-2 game plan almost completely hinged on Pirlo being fit and performing at a high level. Conte went with it because it was the best system for our starting 11 despite Pirlo being old and historically injury prone. Imagine if he opted for a different system that doesnt bring the best out of Pirlo just because we had no Pirlo replacement.
When Pirlo was unavailable (thankfully he never had a long absence) he used Marchisio in the Pirlo role as a temporary adjustment and then made tweaks in how the rest of the team functions now that we didnt have Pirlo's incisive passes. It wasnt a 1:1 change where Marchisio was asked to be Pirlo and everyone else pretends Pirlo is on the pitch. Allegri later used Marchisio fully in the Pirlo role, and then later changed the formation entirely because Pirlo was leaving and marchisio had different qualities (stronger defensively, weaker passing range and ball control) requiring the other players to function differently.
This is what Sarri should do. Use the gameplan that brings the most out of your best 11 players. If someone becomes unavailable then start making tweaks to make a variation of it work. If a key player has a long term injury or leaves the club then that tweak will evolve into a new permanent game plan that brings the best out of the 11 players available (after the injury/departure of the key player).
I'd find it very odd if he is working towards a gameplan that best fits his bench as if Ronaldo, Dybala and Higuain are notoriously injury prone or on their way out. I'd also find it odd if he complains about depth with Mario on our payroll.
But reading that article, it doesnt say he is worried about depth, it says it works best with teams who have a specific tactical setup. Where do you get the idea he doesnt use them because of the lack of depth? Or is that more your concern than his?