the bond buying program's key aim is to buy gov bonds and so supress yield
so the 'running out of govt bonds' is only a factor if that - in running out of govt bonds to buy - those bonds start to rise in yield.
theyve widened the remit to include corporate bonds so they can keep driving down yield across a wider spectrum - but the 'real bubble' issue isnt showing itself as yet because the core yieldsthey are targetting have stayed supressed
not sure what they do if and when that stops occurring. but if $40Bn Euro doesn get spent because they have no tranches to buy, i dont see how thats a problem in the immediate now.