Mainframe applications provide solid transaction processing with a high degree of scalability and predictable costs. By exposing 1st class APIs via middleware, you can evolve the front end and back end independently.
ESB on prem or cloud provide API integration with CICS/IMS transactions. An alternate approach is to deploy Liberty to CICS-based applications to enable a Java layer with REST/SOAP capabilities in the CICSPLEX.
Modern JS front ends with well defined back end APIs leveraging transactions from CICS/IMS provide a robust, scalable maintainable Application Architecture.