Not true! Any form of museum, expo, church .... etc. wouldn't allow the use of flash gun. Hence, it's entirely possible to shoot in very dim light where iso3200 or even 6400 is used.
Perhaps you have odd museums, expos and churches in your country. I have certainly used flash in all those situations in the UK, where I live, France, Netherlands, Germany and Sweden where I have lived and worked.
But that is besides the point - which you saliently have missed.
What I am saying is that if one has restrictions on that kind of photography in ones country and if that type of shooting represents a large proportion of the kind of photography that one does then yes, sure, acquire the kind of camera that handles those difficult scenarios well and that will probably not be a D90.
However, if one is a more general kind of photographer - children, animals, scenery, social occasions then a D90 is a very fine box for hardly any money.
Here, for instance is one from the Louvre, Paris - bit grubby, I admit, I only had a point and shoot with me - but with flash. Pas de problème, comme on dit à La France.
Venus de Milo, Louvre museum, Paris. Panasonic Lumix
So, it is your statement that is not true.
QED