Well, through a takeover Optus could capture AYS' margin and so...

Well, through a takeover Optus could capture AYS' margin and so improve their own profits, which becomes more appealing as Optus and the other MNOs are losing out to the MVNOs as the following recent article highlights:
https://itbrief.com.au/story/network-operators-are-losing-out-to-virtual-competition

One particular interesting implication is:
"Although the majors still have their network used they are disadvantaged by the rise of MVNOs because they lose direct contact with the customer and therefore lose the ability to add value by charging higher prices and increasing margin which is taken by the MVNO."

What the article omits is also the ability to cross sell other services that the MNOs lose because of the MVNOs who actually avail themselves of the opportunity to cross sell and strengthen customer loyality.