In this interview by Hugo Oliveira, Álvaro Siza presents his ideas on the link between obsolescence and quality in architecture, and the role that a design's flexibility plays in this relationship. He argues that the convent is perhaps the best example of a typology which is both fit for purpose and very flexible, allowing myriad other uses when its lifespan as a convent has ended. He also laments the current tendency to design a building for a very short period of time - intended to last only as long as it is needed for its original function. He links this tendency back to the Futurists of the early 20th century, where the idea was that "each generation makes its own environment which is later destroyed", an idea he dismisses since "it also allows you to build badly because it only needs to last twenty years".
You can see how Siza creates this flexibility in his own work by looking at his past projects featured on ArchDaily: