Tony Vargas
Legend
Oh, the DM had final say in 4e (no pressing need to exercise it, all that often, perhaps) and even 3.5 (though, it seemed, there was excessive social pressure to cleave to the RaW, Rule 0 was a thing).Not part of the original conversation, but unlike 3E & 4E (where there are detailed rules for DMs), 5E has returned to the OD&D and AD&D roots, where the DM has final say.
In 5e, the DM often has the only say. The rules offer nothing so it's all on the DM, or the rules are ambiguous enough that they must first be interpreted, by the DM, judged applicable, by the DM, judged good/bad or indifferent by the DM, and over-ruled temporarily or replaced with a variant by the DM, then the results of applying said rulings narrated by the DM.
That's some serious Empowerment.

Well, the upshot. Time, practice, & experience make you a good DM in other ways, too, not just good at dealing with an inadequate ruleset. It's a winnowing effect, you get mostly good DMs DMing (and, of course, some determinedly bad ones) that way, as those not otherwise up to the challenge usually figure out which side of the screen to stay on.The negative of this is that like back in the day, being a good DM takes time, practice, and experience.
A "DM Mentor" made all the difference, to me, back in the day.Back in the day, part of the job as a DM was to seek out potential DMs to teach. You'd guide them and help them learn the rules, how to tune their style, and how to adjust on the fly.