It's a good thing Drupal does not implement every new trend right away. Slowly but surely Drupal adapts to these trends and strikes a balance between all these things. Ok, it's a bit slower. But once implemented in the wider Drupal framework, the capabilities of the previous 20 years of work connect with it. This balance between being cautious and ambitious is what earned Drupal a lot of trust.
Most organizations are not ready to innovate like a startup. They count on the Drupal community to adopt the best new trends and to drive innovation for them. And in doing so Drupal also stays open. Moving to pure MACH-based SAAS for the sake of innovation and sacrificing freedom is a tradeoff not everyone is willing to do. In a way, you could say Drupal adopted MACH while staying open. Open MACH you could say.
So yes, pure SAAS might be more lightweight and less costly but can it provide security, customizability, feature richness, openness, and scalability like Drupal does?
Pure Headless stacks provide innovation. But they lack security, customizability, feature-richness, and scalability only putting your business at risk as a standalone strategy. Drupal strikes balance.
DIY Microservices Stacks power innovation and afford maximum customizability. But run and operate costs to achieve enterprise-grade security and scalability put organizations at risk of being left behind. Drupal strikes balance.