Your first problem is use of the term "require."
We do not "require" the current hashrate to secure the network. The hashrate is a byproduct of miners wishing to get bitcoins.
This is true
now. The current causality is:
Inflation schedule set in stone -> Total revenue to be made from mining -> Network hashrate in equilibrium
But in the future, when inflation is negligible and we only have tx fees, it will look more like
Security is required -> Protocol rules make transactions scarce -> Fees are paid for txs -> Total revenue to be made from mining -> Network hashrate in equilibrium
So the need for security will, by one mechanism or another, affect the network energy usage.
do you think that we require more hashing power to secure the network simply because there may be more people using bitcoin than we have now, and if so why?
Security means: Honest network hashrate is greater than the hashrate an attacker is expected to be able to muster.
If more people use Bitcoin, the economic impact of Bitcoin is greater, more can be gained by attacking it, an attacker could spend more resources in order to attack it, more hashrate is needed to be secure.