[volkl48]

Also, most of the busiest places can be visited as a pretty quiet experience in their offseason.

I've had the Grand Canyon, Zion, Arches, Yosemite, Sequoia and others as quiet, near-empty experiences in the past couple years...just showed up in mid-winter (and not on a holiday week).

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Of course, doing that means: Spikes are a requirement for most hiking. The desert may not get feet of snow, but a couple inches of ice on your narrow trail is an issue. Scheduling flexibility is needed because snow/ice storms close roads at times, and fog/other weather can limit the views you're there for. Some places require snow tires and may require you to have chains.

Not every trail/trailhead is accessible, although in some cases different winter-only routes exist.

But, if you're looking for visiting the "bucket list" places without their usual crowds, it's worth considering.