I don't know why you'd conclude that we're going to be worse off than older generations. Dementia is multifaceted, and associated with a lot of variables, including some that have decreased over time.

Younger generations are far less likely to smoke, less likely to be exposed to concussive or sub concussive head trauma, and less likely to be exposed to loud noises that cause hearing loss. The standard of care for cardiovascular health is different, too, and faster interventions with statins and GLP-1s, greater availability and adherence to vaccine schedules (especially the shingles vaccine), all may result in much better old age health.

Will that make up for greater exposure to other factors that make us less healthy? I'd actually think so.