>Why not?

A lot of the debt is based on real estate and physical goods. Realistically there exists a very finite amount of land (and a concrete limit with current materials on how high you can build) and raw materials. Without becoming a space-based civilization that can easily strip mine other planets and the asteroids... only so much of this debt can even be generated unless we move to some sort of society where everyone is like "I bet you 5 million dollars that bird drops a load on that car in the next 2 minutes" hey the bird didn't, pay up "hey man, can you just add it to my tab?".

At some point you run out of goods or land to sell, at which point you have debt holders that want their money and debtors that can't buy more because they are now out of work from manufacturing the goods they, or their peers, were buying.

Part of the reason we have such a disparity of wealth is because the owners (and lenders) of the resources and sellable goods are selling those things to the people making them. Instead of buying furniture from the local furniture maker, you buy Ikea. Instead of going to the neighborhood cobbler or sandlar you log on to Zappos or go to Finish Line. Instead of going to your local butcher you go to Kroger or Publix and buy meat that may have come from a massive corporate ranch thousands of miles away and in the case of ground meat might contain bits from a dozen animals.

We've basically become somewhat like a 'company store' society and many people carry debt for just their consumer purchases.

"Hey I want the new ninplaybox 720 so I can play MaddenBall 2099, charge"

"Hey I need a car, hey that one is 1.2x my annual gross, let me get a 6 year loan please"

"Hey I need a place to live, oh look this one is only 4x my annual gross, let me get that 30 year mortgage with 10% down that I'll probably refinance and/or get another mortgage and either spend 50 years paying off or move in 4 years and basically walk away with no equity pocketed".

None of this is sustainable and it certainly isn't scalable for the few billion people currently not living this way. In 2016 China had 289 million registered drivers, in 2017 they had 316.58 million registered drivers... that's 27.58 million new drivers and even if only 10% of them purchased a vehicle do you really think they paid cash or do you think the bulk of them took on debt? So 2-27 million new debtors in a year. Not sustainable.

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>If I'm 40 years old, I'm saving for the future. I consume less than I produce,

You spend less money than you earn.

If you eat meat and aren't considerably overweight, it takes about 1/2 of an acre to produce the approximately 1996 pounds of food you consume a year.

The average American consumes around 500 gallons of gasoline a year.

The average American's household, work and transportation energy equals about 15,000 pounds of coal a year.

The average American throws away approximately 185 pounds of plastic a year.

The average American throws away about 4 pounds of trash a day.

Without very cheap labor, automation, machinery, relatively cheap fossil fuels, government subsidies, etc become impossible and as all of the things (including resources) involved in enabling our lifestyles they are wholly unsustainable over any long-term time scale.

With debt fueling a large part of this, increasing debt is also unsustainable.