The economic argument for application modernisation in the data centre is that legacy systems cannot handle today's mission-critical applications and face a dual challenge: losing competitiveness or replacing expensive and time-consuming systems. Application modernisation provides a solution by leveraging real-time NoSQL multi-model data platforms, which allow businesses to manage their legacy applications as an asset to be improved and augmented rather than replaced, keeping costs under control by increasing efficiencies and reducing server usage. The use of JSON document store is particularly effective in this context, as it is ubiquitous, growing in use, and supports real-time data processing and AI/ML algorithms. By modernising legacy architectures through three stages - refactoring applications, optimally storing and querying data, and intelligent data processing at the edge - organisations can lower their Total Cost of Ownership, reduce disruption, and improve business value and scalability. Continuous application modernisation is essential for delivering at scale, improving customer experience, and laying the groundwork for future growth.