As the CTO of rConfig, I've devoted over 14 years to crafting solutions that empower network engineers to navigate the complexities of automation effortlessly. In today's landscape, building network automation applications necessitates a unique fusion of creativity, science, art, and technology. However, the journey from concept to a robust, sustainable application is fraught with challenges that often remain unseen.

Developing an application demands not just programming skills but a persistent commitment to blending user-centric design with intricate technological frameworks. Network engineers, often proficient in scripting for automation, encounter stumbling blocks when attempting to elevate their solutions into sustainable applications. The crux of this challenge lies within the chasm between coding proficiency and seamless user experiences.

One fundamental stumbling block I've witnessed is the need for a user-friendly interface in automation scripting tools. Engineers grapple with complex configurations that demand intuitive interfaces to simplify intricate tasks. However, this is often the point where frustration sets in; the transition from scripting to application building becomes daunting. Despite the array of software tools available, the lack of guidance on how to transform scripts into polished, functional applications creates a barrier to progress.

My journey from infrastructure guy, to software engineer onwards to CTO has underscored the vital essence of simplicity and ease-of-use in applications. Yet, building an application that transcends mere functionality requires more than programming prowess; it necessitates an acute understanding of design principles. Integrating design thinking into the development process is the linchpin to crafting applications that not only work but seamlessly fit into the user's workflow.

For network engineers venturing into application development, bridging the gap between scripting and building applications hinges on embracing design principles alongside coding expertise. A paradigm shift from mere functionality to usability defines the trajectory of successful network automation applications. It's this amalgamation of skills that propels us toward crafting sustainable solutions that stand the test of time.

In conclusion, the challenges of building usable, sustainable network automation applications are multifaceted. It requires a holistic approach that amalgamates technical acumen with design thinking. At rConfig, we continue to champion this blend, empowering engineers to transcend the barriers and craft applications that redefine network automation.

Remember, the journey to mastering the art of application building is not just about writing code; it's about weaving together the intricate tapestry of technology and user-centric design.