Then we should allow people to make various choices, depending upon their age and the "investments" they have made into the Social Security system. For example, all people now younger than 30 would simply be placed out of the system, and enabled to set up whatever private system they choose from this moment forward. In addition, anyone older than 30 who so chooses would also be entitled to opt out of the Social Security system for a one-time payment based upon a prescribed formula. And other people, depending upon their ages, could opt to stay in the system completely and receive the "promised" benefits when they reach age 70, or stop additional payments, and receive reduced benefits at 70, in addition to whatever other benefits they arrange for themselves privately.
But superseding all of this must be the understanding that we are adults, and that the government is not our parent. If some people want to have additional money to spend now, and pay for it by continuing to work late into their 70s, they should be able to make that choice. Furthermore, government should not have the ability to take our money from us, pool it in whatever manner it wishes, and give it back to us when and in whatever amounts it decides.