If he were to suddenly be gone at a crucial window in time it could be very bad. I'm not talking about Succession shit, I'm talking about the fact that there would just be a natural disorganization/chaos that would happen when it comes to decision making in various departments during the interim. There wouldn't be the cohesiveness in the game plan that he currently brings (for better or worse) and the synergy would be off.
A 55 year old GM could have a sudden heart attack and drop dead on the job. The same chaos you fear would still happen. I'm 99% sure Lou gets his physical at the same time the players do--in September at Northwell Manhasset. I know for certain the players get it then because my cardiologist is on staff in the cardiology department at that hospital and he saw them at the stress test lab last September, and Lou was at the hospital that day as well.
The other thing is what I mentioned earlier, just that he won't be around to have to deal with the consequences of whatever he's doing. I'd feel much more comfortable if there was a clear cut plan for his inevitable departure. Maybe there is and we just don't know it yet.
I'm sure the owners have that plan in place. On the face of it they gave Lou carte blanche to run hockey ops, but I'm sure they aren't totally in the dark about what they left him alone to do, at least privately anyways.
As far as succession, I don't think it's nearly the problem you're saying it will be. It can go in 1 of 2 directions. Either the owners want someone to continue on Lou's path and they'll go with Chris Lamoriello. He's an organization guy and I believe is aware of what his father has set up for the future. Or the owners want to go in another direction and hire a GM from outside the organization, in which case there would be sweeping changes to the team, coaches, scouting, minor league ops.