That's the problem. It's not a spending problem per se, but the market's misallocation of resources. Too many resources are concentrated in too few hands.
That's not to say we should abandon capitalism, but rather we should change the incentives to support higher incomes more broadly, as we did in the 1950s and 1960s. It's not a coincidence that deficit spending/public debt skyrocketed when we cut the top tax brackets. That changed the incentives from encouraging paying good wages and investing in business growth to hoarding capital and the financialization of everything.
Feel free to disagree, but the historical numbers support that.
tl;dr: change the incentives to broaden the distribution of resources across the economy, strengthening the economy (70% of which is consumer spending) and increasing, in a broad-based way, tax revenues.