Mining is required for more than just the fees or the block reward. Even if fees were zero and block rewards didn't exist, people who had bitcoin would mine so

Mining is required for more than just the fees or the block reward. Even if fees were zero and block rewards didn't exist, people who had bitcoin would mine so that no one else could mine and trivially double-spend (and thus devalue their bitcoin). The value of mining is based in no small part on how valuable bitcoin are since mining is what preserves that value by preventing attackers from destroying it. What's worse, the needed amount of electricity to secure an amount of bitcoins is relative to how much energy an attacker can expend, not some constant value which will remain low.