That's plastic welded...as in it was originally a plastic post that was heated and basically melted into place. Those sections can be cut off using a large drill bit centered on the weld. The only way to reattach after that would be epoxy maybe?

 

I was going to attempt that but on my original door panel it has carpet on part that price attaches too and I fear that appoxy wouldn't really hold it

 

Then you need to find a complete matching color door panel to put on.

 

I agree this would be safest way to replace it. It could cost an arm and an leg, but it will surely solve it permanently.