Constitutions work like anything else in politics: if nobody acknowledges their validity, they might as well not exist. In other words, if nobody opposes voiding the constitution, it can be voided regardless of what it claims.
Of course most countries with such provisions have some systems in place to prevent this from happening (e.g. by having a supreme court that is legally bound to act in the spirit of the constitution).
Countries ceasing to exist is actually an interesting topic.
Germany, for example, is generally regarded to have been founded in its current form after WW2 when it received its Grundgesetz (literally "foundational law") that effectively became its constitution. The Federal Republic is not the same government as the Third Reich.
However a few decades later the Democratic Republic (East Germany) dissolved and was absorbed by the Federal Republic. So cities in East Germany were formerly German under the Third Reich, then German in the Democratic Republic and now German in the Federal Republic.
Additionally the Third Reich is called "the Third" because there were two other empires preceding it: the Weimar Republic (1919-1933) and the German Empire (1871-1918). So if we consider the Federal Republic as a successor to all three of these, does that mean Germany only existed since the foundation of the German Empire in 1871?
Well, no. Germany as the political entity we would today recognize as Germany indeed began in 1871, but it was preceded (with a few years of chaos in between) by the looser German Confederation (1815-1866) which in turn was preceded by the much larger Holy Roman Empire (962-1806).
If you try to dig any deeper than that you end up with various kingdoms incorporating separate parts of what later became Germany and a lot of parts we wouldn't consider German, so at that point the term "Germany" loses its meaning.
But in all that basically means there are three separate entities we could think of as "Germany":
1. The Federal Republic of Germany, which is an internationally recognized country today.
2. The internationally recognized nation of Germany, which has existed since the foundation of the German Empire and which since the reunification is represented solely by the Federal Republic.
3. The socio-cultural entity we call Germany, which dates back to the Holy Roman Empire (and, in parts, even beyond).
Also consider the role of Austria during WW2: because Austria had been annexed by Germany ("voluntary" or not) it effectively ceased to exist as a separate entity. However Austrians would very much disagree if you said that Austria as an entity ceased to exist as well (and later popped into existence again).
It gets even more complicated if you consider all the various parts that were at various times part of "Germany" (in any of its forms) or not.
At the end of the day, a country is whatever other countries agree to be a county and the cultural, geographic and political definitions tend to not always line up.