Ann: AGL: Enters binding agreement to acquire Click Energy Group, page-26

On the potential fallout of the energy divestment we should have clarity in 29 days when the deal is expected to be fully executed.

Not sure where exactly you take your numbers from but I see "Network and wholesale related expenses" for FY2020 of $112.393m that I assume go entirely to Optus.
If AYS switched to TPG and this cashflow would go to them instead of Optus, would you agree that Optus would have $112.393m less income?
How much do you think is the incremental cost of Optus to serve the 831k AYS subscribers on top of its 9.7m other customers and in return how much expenditure could Optus save if they suddenly had 831k subscribers less?
Do you think they could...

• dismantle 7.9% of the radio towers?
• cut off 7.9% of the antenna panels, RRUs, servers, software licenses etc. and return them to the vendor?
• reduce transmission power of all towers by 7.9% to save a corresponding amount of energy?
• you dig out the tousands of fibre cables running to the towers to split off 7.9%?
• you could fire 7.9% of your network operations staff because the traffic is lower?
• leave the Network Operations Center unmanned for 2h per day because there are 7.9% fewer customers?
• that they could return 7.9% of their frequency spectrum to ACMA?
• you could reduce your liabilities by 7.9% and the corresponding financing cost?

I hope these examples make clear what I meant above when I wrote that in mobile network infrastructure is not dedicated to individual subscribers but shared by the entire subscriber base. You have huge fixed costs which come from building and maintaining a network including thousands of towers in order to cover a certain percentage of the population and a certain share of the country and the more subscribers you add, the lower go your unit costs and the COGS.
I claim that if Optus lost the AYS subscriber base they could not save more than $20m-30m p.a. and I think that this is still optimistic. The incremental cost of serving AYS subscribers is close to zero and hence the saving from losing them would be as little. The $112.393m AYS paid to Optus last year went directly to their bottom line and if they lost them, it would hurt them badly as it would cost them an estimated $80m-90m of EBITDA.
The economics of mobile networks are very similar to software vendors - they have a certain cost to develop an app, you need x sales to break even and from that point on every additional sale is pure profit. That is the crucial point you miss in your calculations.

On your other points:

1) AYS did not show significant EBITDA erosion and because they don't have corporate customers, they're quite immune against the same.
2) Particularly because of 5G the network upgrade cycle has restarted which is why MNOs need subscribers to lower COGS and fund the CapEx
3) TPG can't afford to wait and watch how the other two roll out 5G while they lack the cash flow to follow - they are under massive pressure to scale up quickly.
4) How does their own valuation affect how they valuate a takeover target?
5) As in every market there's a segment of "young, internet-savy, price-sensitive" customers, as you describe them, and any MNO needs an instrument to get a piece of that cake, too. They might not be particuarly profitable and loyal but you can't afford to leave that revenue to your competitors.
6) The migration risk is quite moderate as @goosmurf already pointed out above when he explained: "TPG migrated 100k+ customers from Optus to Vodafone previously, and did it well. This was noted on the 21 August 2020 TPG results call when Nick Harris of Morgans asked if iiNet's MVNO users were still on Optus and if they would be migrated to VHA."
The fact that by June 2022 there will probably be a significant number of subscribers with eSIM-capable handsets will facilitate a mass migration as AYS would not need to swap SIM cards but just provision a new IMSI over the air. And if AYS are smart they will start issuing Multi-IMSI SIM cards immediately after signing a new NSA with TPG. That way any newly issued AYS SIM card will have an Optus identity and a TPG identity and on 30 June 2022 a silent text message will make the SIM cards switch automatically from one to the other identity.