... and It is ok.

I bought 16TB disk because I needed that much storage.

If I were using, say, only 10TB, I would be wasting 6TB.

I can expand my LVM as much as I can. But I definitely want my disk to be full. Same goes for my RAM.

And I want my CPU ide time to stay as close as possible to 0% and my load average as close as possible to my no. of CPUs.

Anything else would mean oversizing and overspending.

Slack has value, though. Your full disk is likely to be very fragmented, making file access slow. A CPU running a full throttle cannot deal with spikes in demand. full RAM means less cache space available. And these issues get asymptotically worse as you approach 100% utilization (which can have catastrophic effects in organizations which are too heavily optimized for high utilization).