As an oenophile, I would say it's about balance. Like a good wine (that balances sugar, acid, tannin and alcohol levels) a good stereo is made up from matching components ito quality and cost. E.g. if you're in the Larry Ellison league, by all means buy the $80k+ DCS setup to match your Wilson Alexandra speakers, $20k Nordost speaker cables etc.
Like the previous poster said, for normal income people, the amp is probably a good indicator of the system value. One could ostensibly justify a $2k - $4k dac in a system where the amp run $3k - $10k, or spending $500 on a dac with a regular headphone amp.
I do agree that DACs are different from other components in their 'shelve life'. Some 10 yr old amps (Mark Levinson, Audio Research, Mcintosh) are still coveted and maintain their (depreciated) value very well (look into Audiogon's Blue book for example). Dac technology on the other hand is rapidly evolving and ever changing.
It just means that the upgrade window on Dacs are typically shorter/smaller than for ancillary equipment. I would venture a dac upgrade every 2-4 years might be prudent if you buy pre-owned and re-sell within that window. That would minimize the loss due to depreciation.
It's all relative. Depending on disposable income for example, one guys' overkill might be the next's entry level.
As always in this hobby, YYMV.
