I wouldn't 'undo it' but it's a much bigger short term risk than most everyone makes it out to be.
The talent that Subban brings I don't question. I don't worry overmuch regarding his personality being a culture issue as well since management has been very transparent regarding their acceptance of his personality, most of the team is of a younger generation, and leader of the old guard (Fisher) spends enough time in the "entertainment" world to work with it if it happens to bother him.
I'm a bit more worried over how that top 4 is paired together.
Weber-Josi worked pretty well together in that it had such a physical presence in Weber to help break a cycle and Josi did the puck moving. Unfortunately, those roles were
so siloed, opponents keyed on Josi (plus other issues with matching forwards).
With Ekholm-Ellis, both were good puck movers
and Ekholm was physical enough to break opponents cycling on them. Ellis is very limited in ability to be physical.
If Subban realizes that he has play that physical role with Josi, his pairing with Josi will work well as it makes them an even better version of Ekholm-Ellis. Again, Josi is limited in his ability to be physical with opposing forwards.
Subban-Ekholm would be very good pairing, but I'd be extremely worried that a Josi-Ellis pairing would be penned into their own zone too often.
I'm not too hopeful of the results if anyone else plays in that top 4 for a long period of time.
(I have no comment about "undoing" the trade. It's done, so I'm going to focus on the Predators line up as is at this time.)