Idiom : What is the idiom for leaving the one thing because you see a new one? In my language (Burmese), we say leaving a grilled fish because we saw a raw one, which means that people want the things that they don't have, even though the one they currently have is better. I couldn't find a similar idiom in English.
The following come close.
Other circumstances seem more desirable than one's own but in reality are often not.
It is preferable to have a small but certain advantage than a mere potential of a greater one.
It is better to have a sure thing now than a possibility of more later.
Similar to @ELMOJO's answer ...
How about chasing the next shiny (new) object?
If you google chasing the next shiny object, you'll get many hits, most concerning Shiny Object Syndrome.
From canwilldone, for example:
Shiny Object Syndrome comes in various forms, but my variety comes in the form of dropping what I am doing now, then chasing after something new and eventually dropping that too. It leads to a feeling of doing a whole lot of starting of a whole lot of different things, but never seemingly completing anything.
This seems to fit the OP's question, taken to an extreme.
Sounds a little like the Shiny Penny Syndrome.