Old people don’t produce and are typically on some sort of welfare (pension, state medical insurance, etc.) Welfare is demand, but not the type that makes an economy competitive, since its purpose is to maintain a none productive sector of the economy. It’s the same reason printing money & giving it to people to spend however they want causes inflation and doesn’t actually strengthen the economy - Biden learned that the hard way.
Economic systems have a single purpose and that is to motivate high quality productivity from working age people. If you have no working age people, no economic system can save you. Robots unless completely autonomous (in which case they displace humans) still need to be operated and motivated by people. If all those people are old you’re not going to get much high quality productivity.
Same reason why 70 year olds don’t found world beating companies despite having more wealth & so in theory better economic position to become entrepreneurs.
What you said of old people spending is not entirely true. Tasmania, a little state in Australia, has the highest proportion retired people. Older people from the mainland come here to retire, due to the clean ,safe, easy and relax life style. There are lots of natural marine reserves and national parks. Good fishing all around the island. Some live on pensions and many on superannuations ( compulsory government supported life saving schemes during their working life). As a result, real estate is still booming compared to other states, elderly care business are booming such as carers , private and public medical services and facilities. Travelling, camping, car rentals and hospitality are progressing along. Pharmacy, airbnb, and Supermarkets are making a killing. Tasmanian has the highest boat ownership per percentage of population, a lot owned by the elderly especially small recreational boats. Many, even though retired, still own one or more rental properties and other investments. The state has one of the most stable and sustainable economy within Australia even though Tasmania has very little manufacturing or mining industries. Tasmanian do have some fish farms and export agriculture products
All these older and retired people, rich and poor ,do make positive contributions to the economy.
Of course there are also downsides. This state will not be conductive to the young and the ambitious due to limited choices/prospects and relatively sparse population. Most young people will migrate to mainland Australia for better career opportunities and for a more hectic lifestyle . Many overseas migrants choose Tasmania (relatively easier visa conditions )as the stepping stone for the length of their compusary stay before dispersing into the mainland. Win some, lose some.