"We may be occupying less than 5%, but our impact far exceeds this number, especially on healthy natural lands: "Tobler et al. partitioned 217 countries into a total of 19,032 polygons with 295,000 persons per polygon. This amounted to an area of 25.4% of earth's surface. So that would be around 96% being considered as occupied at a population density of 42.45 people/sq.km (But the quadrangles would also include a lot of protected and unprotected wilderness and uninhabited deserts. So it would be far to smoothed to give a proper idea of how much land is inhabited)
_{by Tobler et al. (1995)The Global Demography Project (Natl. Center for Geographic Information and Analysis,Univ. California, Santa Barbara) , Tech. Rep. 95–6."*}

Furthermore...

"Another interesting perspective was given by a new global map of European Commission's Joint Research Center which was published in the World Bank's World Development report in 2009.[3] According to the report, 95% of world's population is concentrated on just 10% of world's land surface. While only 10% of the world's land is classified as remote or more than 48 hours from a large city. So we can either state that most humans now inhabit only 10% 0f world's land, but we have the rest 80% well connected with roads, highways, farmlands.etc. So depending on time taken to travel, we can say that humans now inhabit 90% of world's land leaving only 10% for wilderness."

For the moment, most of the material used to make high rise is in deed underlined in your note just above, but loads of new materials are arising and finding their way into structures of all sizes and 3-D designed printed. According to the researches made on the use of skyscrapers of all sort, we could use such building strictly for agricultural necessities and water could be either brought up and/or gathered as well. Many options of energy alternatives are offered and already running on the front. the buildings themselves wouldn't need to be containing any offices. the use of wind gathered within the buildings' structures allows for electricity generation and thus providing for all energy consumptions necessary for water/air pumping, when needed and where moisture, water and heat is needed as per arcology principles, see Arcosanti publications.

Great conversation, by the way, thanks a lot for this as it is such an important topic we need to tackle as a civilization.

Namaste :)