Na don't get me wrong I think mirana is a bit OP right now, and mirana has been plenty buffed since then, but dota is definitely a game of trends and more complex than just a list of +'s and -'s.
PL was hailed as the most overpowered carry ever seen last year, but do you really think a -5% damage nerf to his illusions is what completely killed him? Not at all, his counters got significantly buffed, people learned to play against him, KOTL (his buttbuddy for life) got nerfed, and tranqs (his only decent boot choice before travels) got butchered.
Naga was called a weak carry by pro's and pubs alike after her TI2 nerfs, and she went unplayed for a year, and then played as a support for another year, and then out of nowhere you have carry naga surging back even though she'd been nerfed even more. Don't think anyone, including myself, ever anticipated Lycan becoming a first ban or pick any time soon despite his buffs. Let alone howl being called his most overpowered spell, lots of people considered it to be his worst.
Trends, trends, trends. Pro's and pubs alike tend to get comfortable with certain heroes and it's hard to break that until someone has the balls to discover something new or icefrog slaps a hero with plenty of -'s, overpowered or not (or the opposite, +'s for a hero that might not necessarily be underpowered). Even if dota reaches some hypothetical perfectly balanced heaven, patches to shatter the latest trends will always be required.