If man continues to scathe all of Earth and its seas, then this isn’t the world I thought it to be
make sense? Or should I just replace the 'to' with 'would'?
I completely agree with Kris.
It is grammatical and comprehensible.
I thought it to be translates into I thought it is/was.
Would has an element of prediction or hope which is not your intention I presume.
To is more assertive and definitely well suited.