How is it useful now? Breastplate is 14+2 dex with no disadvantage? SO they are allowing heavy armor's to be mithril too perhaps? I don't have DMG so i'm confused. Studded leather is 12+dex so a 4 or 5 dex can get the same AC as Half-plate. Mithril version of half-plate I guess would be nice for medium armor stealth types with low dex? Heavy armor needs str minimums so fits a unique niche already.
At that point if you want medium and stealth you might invest in the feat that DOES allow +1 more to dex for the armor and also removes disadvantage. Just surprised it doesn't boost the dex max since a feat does and ALSO removes disadvantage anyways.
Seems like they just made mithril a very lesser thing in this edition. I thought a mithril breastplate with +3 dex would be not broken and with the feat allowing up to +4 dex would be something a really high level rogue/valor bard, dex fighters, etc would be great. Almost as good as heavy armor but still not quite AND it required a feat and "mithril" to do it.