One commonly perceived problem is the increase not just in the fraction of the population that is retired, but how long they will remain retired if provided with adequate care, or (more bluntly) how long it takes them to die compared to previous generations. Here in Germany the social system was established when most people died within two-three years of retirement age. Today they can expect to live for more than fifteen years, thanks in part to more expensive health care (cardiac arrest used to be effectively lethal, if not immediately then through subsequent arrests, now it's generally survivable with expensive care). The current retirees are entitled to this, having paid into the system, but current workers can look upon their own increasing contributions and the overall maths and feel uneasy that though paying more they may receive less.

The retirement age is now slowly being raised, and given time the system may level, but in the meantime "issues" will indeed present.

Raising the retirement age is one thing, but finding work for those retirees is another. Ageism is rampant (and especially in tech). Maybe that becomes less of an issue when more of the population is older, but I don't think we're there yet.