Think that's a very spinny look at those particular stats.
If we're spinning in the other direction we could point out that our defence had already been getting worse in each of the three seasons before De Gea ever arrived. Starting from when Queiroz (noted for the impact he had on us defensively) left the club and covering a period in which the team was seen to be in decline generally and our defence (not just Van Der Sar) was aging.
Then De Gea arrived and oversaw a defense that conceded fewer goals than the one he inherited in six of the subsequent ten seasons. This despite the obvious massive downgrades on SAF, Vidic, Ferdinand and Evra he was working with throughout that period. A period in which he was then seen as our repeated player of the year, as opposed to the now revsionist take that he was the one actually at fault.
We could also point out that by opting to start in 06/07 you're beginning with an extreme outlier period in which we were unsusually strong defensively. Looking back on the years before then our goals conceded were 34, 26, 35, 34, 45, 31, 45, 37, 26, 44, 35, 28, 38, 31, 33. So not a single example of us having back to back seasons of conceding fewer than 30 goals, let alone the four in a row you're framing De Gea against. In fact we had as many sub-30 goal seasons in the ten years before your start date as the ten years with De Gea. If we're saying De Gea is the cause of us conceding more goals than we did in that 06-10 period, are we also saying Schmeichel was the cause of us conceding more goals than that across his time here too?
Because it would probably make more sense to conclude league goals conceded isn't a particularly good way of specifically assessing goalkeeper impact.