For me, since Max gained an advantage by going off track, the FIA should've asked him to hand over the position to Lewis. Since they decided to do nothing, it's tricky to come back and penalise Max retrospectively. How do you assess what is a fair penalty? A grid pen? Retrospective 5 seconds? Even, if you think Max deserves a penalty, the fact that the FIA didn't intervene has major repercussions.
After Lewis passed Max, the latter cruised to the finish, with his gap to Bottas going from 10 to 3 seconds. Obviously a penalty would impact the result, but then is it fair and in the spirit of the sport to penalise someone so late after the fact, especially when certain decisions were made (like Max cruising to the finish) based on the fact the FIA said there would be no action. Furthermore, now you have drivers like Leclerc, who've come out and said they would race differently, based on how the FIA judged the Brazilian GP. This is bad precedent the FIA have set, and I wouldn't be surprised if we see some outrageous driving towards the backend of the season.
Personally, I would rather the FIA move on from this situation with no penalty, but add more clarification to the sporting regulations. A lot of the decisions are at the discretion of stewards, which just allows for too many subjective takes. I'm convinced they allow for more leniency with drivers that are competing for the win, as opposed to those in the midfield. Tsunoda's penalty at the Brazilian GP is probably the best example of this.