Running the Bitcoin mining network costs millions of dollars per day in electricity costs alone. Who's going to to pay that when the subsidy goes away?

You are wrong about mining costs. Miners are estimated to consume 100-200 megawatt. Most of them pay below worldwide average electricity prices, being located in Iceland (KnC), WA state (BitFury), etc; maybe around $0.05/kWh. So the miners' daily costs would be $120 000 to $240 000.

The block reward only halves, it doesn't "go away". Unless you are talking about when it can't halve anymore because of the 8 decimals limit, which we could just remove, just like Satoshi wanted (same situation as with block size limit).

If we keep the 21M limit, subsidy will be insignificat at one point.

If both transaction fees and the coinbase are zero or negligible, there is potential revenue in mining: to reverse transactions and rip people off.

Altruism certainly exists, but assuming that altruistic actors (rather than self-interested actors) will expend more resources than attackers is a very different security model than what Bitcoin is designed upon.