For what it worths, it's already implemented in C-lightning (the 'exclude' parameter in getroute).

So the question is do users actually use it ? It's been here for a long time and I've only seen plugin which use it (like "pay" to try multiple routes). In addition to users, who the implication in route gathering should (/will) be limited in the future (the aim is autopiloting everything), will the autopilot algorithms have a decentralisation concern ? If yes would they be less efficients than ones without concern ?

I think the only treshold is user incentive : if a "hub" tries to increase fees then an autopilot would try other routes, but if the "hub" is part of the most efficient route (which represents a real operational cost, and a real exposure), then I don't think (to answer the quoted question) that it would improve decentralisation at all.