The fact a part of the island didn't sink, in spite of all the signs and foreshadowing the whole thing was going down right then and there, was way too convenient ? And for Mr.Yamada to have also coincidentally hidden a bag right in a place that stayed above waters is, while not impossible, quite convenient too ?
But leaving these details aside which are needed for the plot : why ask her to go with him 10 years in the future, instead of looking for a fog that could return her home?
I understand that he comes from a timeline in which she is considered as dead, but wasn't his purpose in coming back to the island to save her? ... He wishes for her to live, but he still won't go as far as changing what happened in his 10 years by adding up a person which might influence his whole life and maybe other things...not like I don't understand, but they are acting as if she didn't have a life and a family waiting for her at home, despairing to see her again, or what impact it would have in the future if a person appeared as young as they were 10 years ago! (she is also way too carefree and happy too about the whole situation, but yeah...)
And then what? Go to a future in which Yamada and Chika are a couple?
The ending the author chose for Kamiyama has so many loopholes and loose ends it's ridiculous.
Ultimately, it might be best to simply be happy for her, being with the person she loves, no matter how long or short that time is on that island, and not think too hard about anything beyond that...