Liverpool missed for a point last year doing the double. And even in the case of United, it is very hard to argue that we were better in 2007-2008 than the following season. The only difference that we did the double in one but not the other season, was that we faced Riijkard's Barca, nor Pep's Barca in 2008. If Terry didn't miss, would that team had become weaker? Was Chelsea in 2012 the best team in Europe?
Yes, achievements matter and definitely they are important. A team that wins titles is greater than one who doesn't. But it is necessarily better? I don't think so. Similarly, I don't think that United in 2008 was as consistent than Pep's City and Klopp's Liverpool, so I don't think they would have won the league against these teams. And United in 99 would have finished 15 points below or so. Put this City on the nineties, and it takes the piss out of the league, most clubs would look amateurs in comparison and would struggle to get 20% possession.