Users are not paying for security, they are paying for having their transaction processed. In fact, I don't see that users have any way to measure the security of the network granularly: if secure, everything works. If not, systemwide chaos ensues.

The security of the network can be measured by any user. Multiply the mining difficulty by the efficiency of mining hardware and the price of electricity and divide by 2; this is the cost of running a 51% attack.

The beauty of the mining mechanism is that having transactions processed provides security at the same time. As long as users see value in having transactions processed they will be willing to fund miners with transaction fees.

However, you're right that users aren't paying for security directly. The fee market may not actually give miners enough revenue to ensure network security. When the block reward goes away electricity consumption will certainly drop (relative to transaction volume), but it may even drop too far to the point where the network is no longer secure from 51% attacks.

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.

The final block will be mined in 2140, so we're not quite to the transaction-fee-only point yet.

The block reward in the final blocks will be 1 satoshi (1/100000000 of a bitcoin). The block reward will be completely irrelevant many years before that. We don't have to wait until 2040 for the fee market to overtake the block reward.