If you can’t get into the building during a power failure then I don’t think you’ll be able to use the lift either. It’s a bit hard to know without some further information what the best scenario is for your building, eg how many units and how many floors. If there are lot of units then obviously the more keys given out and duplicated by the residents, the less security prevails. And as you say you have to watch out for the baddies within! It should be quite feasible to put in a battery backup for the security system so it will continue to operate during a power outage, whereas a lift would require a diesel generator, a much more difficult and expensive proposition. Unfortunately some of these issues aren’t well thought out by the builders, and the developers are happy to let the purchasers sort out these shortcomings (at the expense of the Owners of course!)