I do agree that Gucci's hype will last two or three years at best, but I don't think it's because they are following Slimane... It's because people in the fashion industry are waiting for the next big thing faster than ever. There's no pacience anymore. No evolution as you say. A brand does some viral shows, they get every cover and make headlines, then another viral show comes and the cool factor of the other brand is long gone...
I agree also but i think Celine, Valentino & Saint Laurent changed something in the system.
For me until 2009/2010, the industry was still a very Tom Ford era-influenced industry. The designer was so powerful that the consumers trusted him. It was all about having a style rather than an aesthetic. Designers were able to do a 180° each seasons and stayed true to themselves.
Phoebe gave people what they wanted but she was smart enough to introduce new ideas with her main collections and capitalize on them with the pre-collections (a bit like Nicolas Ghesquiere did at Balenciaga). Thanks to that, she is not locked in the "minimalist" box (even if she is not a minimalist IMO).
Valentino is all about an endless formula. It's not about proposing new ideas and challenge people in terms of silhouettes, prints, techniques...Etc. It's much about giving the illusion of new.
Hedi took it to another level with vintage clothes and vintage him. Saint Laurent is in a way totally different from what he did at Dior Homme. Dior Homme was about modernity whereas Saint Laurent is about retro.
Hedi, like Nicolas, Phoebe, Tom and Miuccia got a very loyal fanbase and his clothes have this incredible visual power: even without any logos, even a full black outfit is recognizable.
It is a pity that he doesn't try like he used to. For me Designers at this caliber should challenge their customers while doing something familiar.