"I would imagine that costello ran the economy in a sustainable fashion that managed to pass on to the next government some money in the bank he was probabley seen as too conservative to achieve such an honour. Just got things done without worrying about accolades."
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An alternative theory is he or Howard assumed the rivers of gold would continue indefinitely which is why they introduced ongoing and increasing (due to population growth) middle class welfare which would be notoriously difficult to unwind.
When the music stopped post GFC, the revenue growth slowed and the middle class welfare continued its climb into the clouds.
No flapping wonder the budget position deteriorated quick smart.
Not to forget they flogged off all our national assets which would still be earning dividends for taxpayers to this day if they remained in the boobs of the public.