No one person is to blame.
Ninian - these are valed opinions, however I disagree.
Mackay spent £35.0M or so - what would you expect for this investment? Champions League? Top Half? For £35.0M Mackay assembled a squad good enough to stay in the division, which for that amount of investment is a job well done. Had Mackay remained manager we would be safe by now (only an opinion I know). I agree he was a bit negative, but was starting to use Mutch & Noone after a half decent start.
Hull spent nowhere near that amount nor Palace and look where they are now? Mackay was being naïve to expect his championship squad to be good enough supplemented with one new centre back, one new holding/defensive midfielder, two untested attacking full backs (one whom he never played), one untried striker, and one experienced attacking player (who he then played out of position). The bulk of last season squad have proven to have been inadequate for this league - Gunnarrsson, Cowie, Kim, Tayor Turner, Hudson (albeit just one disastrous game).
Mackay failed to start with Noone until late November - why??
OGS - no blame really - poor guy looked out of his depth from day 1 - CANNOT be allowed to stay for next year.
He has to take blame for his wrong tactics and team selections surely as well as bringing in some Norwegian players who don't look good enough even for the championship?
Tan - where to start - sacking Moody, sacking Mackay at the wrong time - should have stayed 'till end of season, threatening players with withdrawal of bonuses, booing his own players, having total disregard for Cardiff as a city, its fan base, its history, interference in team affairs, giving away Cornelius, when we were blatently short of fire power ( I know he wasn't exactly prolific but come on - 80 minutes playing time in total & Kenwynne Jones FFS).
Tan surely had the expectation his money would have been spent appropriately and not on "punts." I reiterate Hull and Palace were below us last season - look at them now with astute management and player acquisition from experienced PL managers.
Players - as I've said before - squad at begining of season were good enough to stay up & were actually playing well enough to stay up - in my opinion they weren't interested once Malcy's position was threatened -OGS's indecision & lack of tactical know how didn't help either. Can anyone explain why Caulker & Turner looked solid under Mackay, or Taylor actually looked good enough for Premier League, or Medel looked a bargain, or KTC looked an excellent purchase?