I'm not exactly sure how well it speaks of Hedi Slimane that his successor manages to come up with a collection that makes the regular customer at Saint Laurent wonder if the house really changed designers.
One thing that can be said about Hedi is that, once all the blocks of the re-branded Saint Laurent were put in place, it was possible for another designer to pick up where he left and grant the house a steady growth, granted he would follow the mold established by Slimane. You can see that being the case here as much as it was apparent at Dior Homme where the tailoring foundation was laid and a lot of timeless products introduced by Hedi Slimane keep on being best sellers (tailoring, jeans, bags, footwear). Despite some stylistic tweaks, you can see that Vaccarello's jackets from the 1st all-black part of the show were modeled after patterns that existed from Hedi's tenure. Dito for the menswear - No way they would let a designer with zero experience in menswear take on a significant part of the houses' business without the reliance on a large amount of products they can re-issue with small variations. Yes, this is a bit more sexy looking on the runway but I don't think the majorities of SLP customers who are not diehard Hedi Slimane fans from the very beginning will find it hard to adjust to Vaccarello's design.
Rick Owens said in an interview that he believes his namesake company could live on without a designer for a good 15 years by re-issuing or tweaking his existing designs by seasonal parameters, so maybe there is some truth in that here.