When you say the new owners have to maintain their own systems, what does that actually mean? Subscriptions, hardware repairs, regular inspections? Has this been the case since the new apartments were built/bought?
The fact that the older apartments have an integrated system would be a significant factor in favour of using the sinking fund. Perhaps the management fees do/should reflect the different maintenance costs but it would be heading down the road of 'its takes longer to clean these hallways because they are wider, so they should pay more'. Clearly everyone benefits from an modern and integrated fire system.