"Rulings, not rules" does not grant DM's any freedom they didn't already have. Every. Single. Edition. Of this game, has had a large swathe of DM's (if not, eventually, all of them) who have made their own house rules to modify the game.
The DM never had their power taken away by the rules of the game. Never. Not once. 5e doesn't empower the DM any more than any other edition of the game ever has.
You might say "well, other editions have had complex, interwoven rules that made it difficult to change any one thing without causing other problems". To which I reply, yes, and?
To make good house rules, one must understand the purpose of the rules they are changing in the first place.
5e does not make this task any easier. Does anyone really understand why, if I cast a spell as a bonus action, I cannot cast another leveled spell with my action on my turn? When an Eldritch Knight can cast two leveled spells via Action Surge and cast Shield without incident?
If you do, congratulations. I don't! So as much as I'd dearly love to take that rule and strangle it to death, I don't, because I don't understand what it's there to prevent!
As near as I can tell, it exists to hamstring Clerics for no reason, and to keep Sorcerers from being as good as Wizards.
There are many rules like this, not even picking on the optional ones, where playing the game without intending to make a single house rule, but completely by the rules as written, eventually becomes a Sisyphean task!
The only thing that is empowered by this is my headaches.