It's pretty unbelievable but listed on the Kitchen Nightmares fansite it shows only 8 of the 22 restaurants visited in the UK series are still open under the same owners.
The break down;
Season1: 1 Restaurant still open
Season2: 1 Restaurant still open
Season3: 0 Restaurants still open
Season4: 3 Restaurant still open
Season5: 4 Restaurant still open

As you can see most of the restaurants still open are one from the latest season... so it should only be a matter of time before more of them close.

Although not in anyways Ramsay's fault, it shows pretty clearly that being on Kitchen Nightmares doesn't help your restaurants chance of surviving much.. pity.