Besides, given the rate Sea-Life is expanding in the US and expanded across Europe already, you can't deny that they appear to have a successful model as they wouldn't be able to expand if it was unpopular.
In fairness, their expansion into Europe was accompanied by the selling-off of many of their UK aquariums.
My main beef with Sea Life (and they're management-level issues, staff in the aquariums I've never had any problem with) is the massive hypocrisy. As tim b intimated, for years their advertising had their ethical stance stated in no uncertain terms : "Sea Life believe that cetaceans and certain species of shark, specifically Sand Tigers, should not be kept in captivity" (that's from memory so may not be the exact wording, but the Sand Tiger name-check was most certainly there).
This stance disappeared entirely once they acquired the London Aquarium, complete with Sand Tigers, and the Sand Tigers remain to this day.
It just suggests, as tim b says, that they took this adverserial stance to cover fot the fact that they wouldn't/couldn't build the facilities (given that a number of their UK competitors (Deep Sea World, London Aquarium, Blue Planet) did have Sand Tigers, it just seems like sour grapes).
They also have a very patronising habit of referring to any exhibit not for fish or invertebrates as a 'sanctuary' - Penguin Sanctuary, Turtle Sanctuary, Otter Sanctuary. I think this stems from the work some of the centres do with rescued seals, which is very good work and with rescued animals then I can accept 'Seal Sanctuary' is appropriate. But, to take the example of the penguins, all they have done is build a standard penguin pool and bring in penguins from another zoo/aquarium. Calling their new home 'Sanctuary' is both incorrect and smacks of self-righteousness, implying as it does that the animals have been 'saved'.
I'll agree with you wholeheartedly that the experience would not be nearly as enjoyable if all the European Sea Lifes look the same. Part of my joy from visiting aquariums is the uniqueness and individuality of each facility and photographing it. But how many visitors are going to go to more than one or two of these aquariums in their lives? You and I are not the target market.
In fairness, they spend a lot of time pushing the SeaLife passes, so they are intending people to visit more than one. You're right than they're not thinking of people going to all of them, but they definitely aren't banking on people just doing one.
This is not limited to SeaLife. Acrylic tunnels are a huge crowd pleaser, you can't build a new aquarium anymore without one.
The Deep in Hull almost managed it - they only have a half-tunnel (i.e. a quarter-circle of Perspex up against a wall). The problem with Sea Life is they don't have one or two exhibits everywhere - they have whole chunks of the building repeated, with the same exhibit names, in case you missed what they were doing (Bay of Rays, Claws, Kingdom of the Seahorse...). They really do feel like franchised aquariums, contractually obliged to be as similar as possible. Many of them are very good days out, despite this (Blackpool is very good), but a lot of them are made solely from these repeated elements and you only have to have done one centre before to find them rather uninspiring.