Any study on life expectancy while on a feeding tube?

Yes, but answers vary widely depending upon cause.
Children and the elderly have lower expectancy, in general, than middle age adults.
https://www.giejournal.org/article/S0016-5107(22)01937-X/fulltext
There are smaller button tubes that don't catch on bedding and clothing MIC-KEY and AMT Mini ONE. It is the debilitating disease causing the death. The tubes can prolong life for a month or months giving a chance for treatments to restore health. But may not be great value for someone who has no chance of recovery. But some do recover and have the tube removed.
Bless your struggle.