Well if you look at it from technical point of view then, No, it is not unachievable. But if add some of your logic then yes it looks pretty unachievable.
First of all lets understand how sharma achieved this huge milestone???
Reason 1. He can bat for long time without getting tired.
Reason 2. His teammates supported him to go for the milestone.
Reason 3. Faced most of the deliveries himself.
Sharma scored 264 of 173 balls with 33 fours and 9 sixes. Lets see how many overs he batted. 173/6=29 overs. He literally batted 29 overs out of 50 i.e. more than 50% of overs to achieve it. Out of them 42 balls were gone for trip to boundry line.
He scored 264 in 29 overs, means 264/29= 9.1 runs per over. You need to score 9 runs per over to score that big. Even in t20 teams score at 9 runs over in average. So you will have to play t20 in ODI.
After he was past 160 runs, he faced most the balls, as he had supportive teammates. As he was set too instead of scoring runs themselves, they allowed Rohit to bat long, because of which he was able to score that big.
He have ability to keep batting long innings once he is set, so that helped him too. Other batsmen get tired after scoring 150.
So logically, this record is achievable only if any batsman have enough strength to bat 29 overs of a match.
Only if he can bat like t20 by scoring 9 runs per over for 29 overs consistently.
Only if he have enough strength to hit 42 boundaries.
Only if he have supportive teammates to motivate him to score big instead of trying to bat for themselves.
So only if a player can fill this criteria, he can achieve this unachievable record.