Good question.

With a lot of old people, healthcare costs go way up. In practice that means taxes go way up.

(Maybe smart AI can magically cure a lot of degenerative diseases like dementia and atherosclerosis and COPD and osteoarthritis and cancer and diabetes and kidney failure. Let us hope.)

The infrastructure we have (roads, bridges, water supplies, power lines, etc.) need maintenance. With a falling population, a greater percentage of the population needs to be dedicated to these tasks, so career choices get restricted.

(Maybe robotics can help here. Let us hope.)

With a falling population demand for any given product is falling on average.

When the population is growing, there is an implicit cushion for investment. 2% growth means that the population (TAM) doubles in 36 years, making investment less risky. With falling population, new investment is taking market share from existing vendors: competition is zero-sum at best, mostly negative sum.

Every investment is more risky so the rate of interest on loans goes way up.

With falling GDP, wages are stagnant or falling. At present young people take on debt to buy houses and things, partly in the expectation that their wages will rise so the debt gets easier to pay off over time. Falling wages make debt repayment harder, not easier, so people will not take out loans. so sales will be lower, leading to a downward spiral.