You don’t have to make every single floor accessible to make the map interesting. There could be a lobby, a restaurant on an upper floor and a few more floor in between. Take Whittleton Creek for example: It’s a small map and you can’t even enter every single house but it’s still a very atmospheric map. Nobody is going to complain about not being able to explore 30 floors of generic office rooms.
Sniper assassin is still possible. Lure your target to a terrace or roof on a lower floor and then snipe him from above. Or trigger his helicopter escape and then snip ehim while he’s sitting in his getaway helicopter. Maybe the second target even arrives via helicopter and you can kill him with a sniper before he even sets a foot into the building?
The biggest issue seems to be that their current engine is having troubles with elevators and a skyscraper without an elevator makes no sense.