Board Game Arena

Is the replacement settlement power of Doomed Colony mandatory? Reading it makes me think it's optional but it doesn't seem to be

The wording seems to indicate quite clearly that it is optional as it begins with 'may'. I have not had the opportunity to try it, though.

It is indeed optional. On BGA, when you select a non-military world during a settle action, if you had no other way to settle it (not enough credits or enough colonists), the confirmation step will automatically default to replacing the doomed colony. If you do have enough resources to settle it otherwise, it will default to settling normally and the action bar will include a button with the option to replace Doomed Colony instead.

Just to add onto this. If you settle a colony that requires two settlers, you need a second settler from stored colonists.