Didnt like the happy ending

They did erase the records of the missing people as stated during the scene of the secret lab.

If Trent and Maddox reported what happened to the police themselves then, yes, it would be too easy and unbelievable. But they didn't. The reason the police took it seriously, and what started the investigation, was that it was the cop at the resort who actually reported the notebook with all the missing peoples' names. That's absolutely all they need for a warrant to search the entire complex. They'd find the lab, equipment, experiments, documentation, etc, and learn what was going on. Then I guess then the two siblings would explain who they are which could easily be verified via blood work and DNA.

How is it a happy ending? Everyone died but the 2 kids and they lost 45 years of their lives.

They could have just ended it with the reveal that it was all a trial by the company to aid research in medicine, or failing that when the survivors went to the cop. The extra ending I agree felt too silly to believe that it would have been handled in that way.

I thought the scenes on the beach were ridiculous frankly, but the end part at least prevented it from being a complete crapfest.

How could a company possibly prove in peer-reviewed studies that their medicine actually works without revealing their criminal actions?