Right, but how many DMs want variant rest rules, slow natural healing and full encumbrance? Ask 100 DMs what variant rules they'd like in the DMG, and you'll probably get 100 different answers. There are thousands of knobs that could be turned by any DM, depending on what they and their table find interesting.
To me, rather than presenting 10 or 50 pages of possible variants, I think giving them a couple of strong guidelines to help them navigate how to make changes to suit their own needs is more important. Give them explicit permission to make the game theirs, and point them to Indie publishers, liveplays and forums like Enworld for inspiration and advice. That, I think, will be far more valuable in the long run for any new DM.
For example, Dungeons of Drakkenheim has a great way to combat the 5 minute work day in their setting and Critical Role has a system to make resurrection both more difficult and meaningful. Sure you could publish those rules in the DMG, but again, how does that serve DMs for who those specific rules don't help them?