If nobody spends their money then your economy will end earlier than it should have.
People advocating for deflation say they will still spend their money but what they really mean is that they will spend the money on basic living expenses and that is it. They will save the rest. How else would they benefit from deflation? If they spent all their money they wouldn't benefit from deflation as their incomes stagnate or even fall.
Deflation isn't just a fall in prices of goods, it's also a fall in cost of labor because the price of goods is what pays for the labor.
It's pretty easy to illustrate how deflation ruins the profitability of companies. You have a production line that generates $5 million revenue a year. You have fixed costs of $4 million. Your profit is $1 million. As the population gets older, demand for your products goes down by 20%. Now you have $4 million in revenue and your costs went down slightly to $3.5 million. You're making half the profit even though revenue only fell by 20%. Running the production line at below capacity makes the production line increasingly less profitable until it no longer makes sense to run it.
A fixed % reduction in revenue can result in a much higher loss in profitability. Low profitability is a self reinforcing cycle as companies stop borrowing money and the money creation process is being interrupted leading to further deflation that cuts into profitability until you reach the smallest possible economy. That economy would have a GDP significantly smaller than $22 trillion USD but the level of savings didn't change meaningfully.
Despite the illusion of deflation, your money is actually losing value over time as the economy gets smaller because your dollars are just a claim to the output of the economy and if the economy shrinks so does the ability to serve your claims. When people realize that the money they are holding onto wasn't risk free after all it's too late and we get very high inflation or even hyper inflation as people get rid of dollars that have no counter part in reality.