How to fix Abs bathtub flooring being weak?

[Sirius0831]

So the bathtub flooring is weak and feels like you're going to fall through at any moment. I am trying to make it so the bottom is sturdy again. Putting spray foam or mortar down isn't an option since I don't think theirs any flooring below it. Can I put rigid foam board on bottom underneath tub and make it sturdy again or is it supposed to be sitting on wood and is just rotted away?

 

[Breplum]

Only by removing rot and replacing with stable material will you end up with adequate results.