Scott Morrison said the government's budget rules mean any new spending "must be fully offset". The first such exercise, produced by Wayne Swan in his last budget in 2013, showed us returning to surplus by this financial year, 2015-16. Just a few months later, Labor's last statement pushed it back to 2016-17. Joe Hockey's first budget pushed the date back to 2018-19 and his second to 2019-20. Now Scott Morrison's mid-year update has shifted the return to surplus back to 2020-21. This ought to be enough to convince you these ever-confident predictions are not to be trusted. They're mere projections, based on assumptions that soon prove overly optimistic. Any treasurer who endlessly repeats the ideologically blinkered but patently absurd line that the budget doesn't have a revenue problem, it has a spending problem, can have no credibility as a budget repairer.

The 2014 budget was the ultimate demonstration that, while repairing the budget almost solely on the spending side may be theoretically possible, it's not practically possible. Such plans can't be made to stick because they're too unpopular and too aimed at requiring the least able to bear the heaviest burden. It seems Morrison's stopped repeating this mantra, but his replacement rhetoric is no better: "Our plan is straightforward – responsibly restrain expenditure while supporting economic growth to lift revenues." Translation: we're prepared to do no more than match our inevitable new spending programs with offsetting savings – as we did last week – while we wait for the economy to return to trend growth and so allow the budget to fix itself. This tells you Malcolm Turnbull isn't willing to increase taxes overtly but, by the same token, isn't willing to make major cuts in government spending that might cost him votes when he gets his ascension endorsed by the electorate next year. If Turnbull goes to the election without mentioning a plan for slashing spending in his next term, what are the chances he'll do it anyway? Not great.

It's possible Morrison has stopped claiming the budget doesn't have a revenue problem because the plan is to cut back superannuation tax concessions as part of the tax reform package. It's also possible the tax package will involve net savings to the budget, if not immediately then a few years down the track as the revenue saving measures grow faster than the cost of the tax cuts. It's possible, but I won't be holding my breath. It's more likely the political frictions in the tax package will require it to be "budget negative", so that – whatever the happy claims about it encouraging people to "work, save and invest" – it sets back the budget's return to surplus. Last week's mid-year update reveals Morrison to be presiding over a structural budget deficit equivalent to about 2 per cent of gross domestic product, even while he peddles the line that economic growth will get us back to surplus. Don't believe it. By definition, the structural deficit is the deficit you still have after the economy has returned to trend growth. Only if the economy were to be in an inflationary boom might the cyclical surplus be big enough to hide the underlying structural deficit.