if the GSIBs are going to buy a heap more USTs they will need to...

if the GSIBs are going to buy a heap more USTs they will need to hold more equity, which comea via retained earnings, DRPs or capital raisings. The GSIBs have no desire to do any of those. As even leveraged 10x the incremental UST returns will never meet their return requirements (without a big fall in rates) as the leverage consumption crowds out their ability to take on risk exposures (loans), that pay much higher returns for more capital consumption, but less leverage consumption.

The GSIBs are all looking to pay dividends and undertake share buybacks, as they generate better shareholder returns which = better exec paydays.

i guess the key difference today vs 2008/9 is the banks are very well capitalised, the key constraint back then was capital with banks meeting mininum capital ratio levels, now its much stronger, but the constraint is on leverage capacity (balance sheet size) which gets expensive as evidenced by most banks efforts to migrate down the GSIB levels due to the burden on the incremental capital they need to hold per GSIB category which erodes returns.