Constitutions become ingrained in to the very fabric of the country and work as a basis for that country's legal and justice systems; even the inner workings of its very society has a basis coming from it's constitution.
That is something that time simply does not change, as disregarding it, or throwing it out, throws out the pillar on which that country is based on.
If you look at the government here in the US, we have been very successful with a government that is ran from a constitution, and I think that a lot of people are going to want this freedom and good life that we have because of our nations good constitution.
Constitutions are essential for governments in the long run. One cannot keep making up rules and regulations on the fly. If they are established traditionally through construction and building of the foundations of a country, then they should be kept that way. The United States, an idol country, has a Constitution.
I personally disagree ,Mission drives agencies, and the need to deliver better services to customers at a lower cost whether an agency is supporting the warfighter overseas, a teacher seeking classroom resources or a family figuring out how to pay for college is pushing every level of government to look for new solutions.
No, I do not believe that the 21st century will still support governments upheld by constitutions, because the constitutions are becoming a joke. The United States no longer follows its constitution. They call it a living, breathing document, which is just its excuse to ignore the constitution. Most other nations don't even have a constitution to pretend to follow.