Adkins was sacked because he was utterly clueless.
He took a side who finished 5th the season before and finished 11th.
He made awful signings in Woolford, Hammond and Edgar as well as going through with the Sammon signing.
He obviously did zero research on our previous season as he failed to sign a centre half in the close season and didn't get a decent one in until Baptiste. He also brought Collins in from the cold and stuck with him for 7 months despite him being shite.
He moved on lots of players, that is true, but in loaning out Campbell Ryce and Freeman we lost any pace that we had in the side and loaning out McNulty until the end of the season limited our striker options. Whatever you think of McNulty he would have got more goals than DCL or Sammon off the bench in the run in.
Biggest negative for me though was his tactics, bizzare selections, motivation skills (or lack of) and his frankly laughable substitions/ability to change a game. We often seemed to come out after half time half asleep. We never once came from a losing position to win. On several occasions when we had a real chance to kick on into the top 6 with a win we failed miserably, often with a whimper and little urgency or desire.
His worst for me was Swindon at home. One nil up and solid, he changed our entire central midfield - even putting Flynn in the middle and Read out wide which just handed the game back to Swindon. Southend away was bizarre too - played Flynn central but young Whiteman wide. Then when behind he took off all midfielders again for four strikers leaving us totally shapeless following Phil Brown changing their formation. Players also went backwards under him. His big main signing in Hammond who he pursued relentlessly was worse than Ian Fucking Hamilton, yet he continued to extol his virtues and extended his loan deal in January when those wages could surely have been better spent.
So yes he inherited a big squad that needed trimming, yes he didn't have a lot to play with in the transfer market but even considering that, what he did do with the team, as above, was absolutely pitiful and showed not one bit of positivity that made you think giving him time would allow him to turn it round.
So no, I don't think failure to get promoted automatically gets you the sack. But failure to get top 6 while being totally tactically inept, making constant idiotic substitutions, god awful transfer market decisions and not being able to motivate the players to play to the maximum of their ability means, rightly, you will be in trouble.
As a footnote all Adkins failings were exactly what Reading fans had said about him so it was hardly a one off.