Origin is becoming a NBN reseller, not an owner of a...

Origin is becoming a NBN reseller, not an owner of a network.
You've reversed this and said TLS should become a reseller of energy.

Then you've added in mobile network and suggested it would be expensive to own a mobile network. (agreed)
The addition is your own, and use this to justify that TLS should acquire and own energy infrastructure. (disagree)

Origin could become a reseller of mobile services but I would guess it more likely be as a reseller, such as reselling Optus.

Companies do this in order to create bundles of services for customers and then make the switching costs or process to change too hard for customers. That's how they butter you up with $100 offers to join. They expect to make that back.

Businesses should grown from their core offering. I recommend reading the book "Profit from the Core" by Zook and Allen if you haven't, it's great!

IMO a more appropriate example would be a comparison with NextDC, a company that built large Data Centres all over australia and grew to $2B valuation. What better way than to vertically integrate and provide related services, including storage and cloud services.
TLS could have provided co-location and 'Availability' Zones across Australia as they already own a data network.