Isn't that basically the mechanism of how early complex cells formed millions of years ago.

First it was just basic cells ... they fed off one another and at one point ... one cell became incorporated into the other and essentially evolved into an organ of the cell ... like mitochondria inside the cell, isn't it basically thought that it was one it's own organism at one point and just evolved into an organ inside other cells.

Same with the human body. I think the estimate is that we are only about 50% of our own generated cells and the rest is just other beneficial cooperative bacteria our body has evolved to take advantage of.

So the Sarlac taking you in is just incorporating you into it's body for some function and keeping you alive to fulfill that role ..... you just happen to be conscious of it and unable to escape the entire time over a thousand years.