It's perfectly reasonable to require a test match opening batsman to score hundreds. For the reasons discussed above, a player who keeps throwing his wicket away is not doing their job properly when they open.
However, it is unreasonable to criticise Watson for not scoring hundreds when he so often makes 50s. That is what he is so good at and is actually a very useful role to have in the side. The issue with this is that Watson is not an opener, does not have the plan or concentration.
As has been noted, saying that Watto needs to convert his scores is asking him to average 80, which he is not capable of and it is unfair to ask of him when so many others would do well to average 50.
The issue is one of selection, and not finding a proper opener. If you are going to criticise Watson for this, you may as well criticise his selection all together, because you are criticising him for being Shane Watson.
(Which, on reflection, could be pretty fair.)
This is only one of several arguments for Watson to move down the order, and him continuing to face the first ball is yet another example of selectorial incompetence.