We tried that and that's how we eventually got them to switch - 2 years later. The client required a feature to start 'live sessions' with mobile subscribers whenever they were active on the network (made or received a call) and support for multi-level menus. The original architecture was transactional and they understood its limitations. The rewrite used SEDA and we were having latency issues, gc and memory issues handling live sessions with SEDA. So we had to do another upgrade to switch from SEDA to Actor based model that worked. The next upgrade was small and incremental compared to the first one very and we had the system ready by the time contract was finalized.

That's the problem. From my reading of the article, their client was, probably legitimately, concerned with the risk of a cutover to a new and unproven system.

To implement a new system the client always needs to do UAT. That's actually a drain on their resources, and if they have a working system it is far better to have incremental changes made and a regular set if bug fixes than to launch a new system, then find a raft of new bugs or unexpected changes that need to be fixed anyway.

Existing systems allow enterprises to operate effectively. New systems almost always cause unexpected disruption to a business regardless of efficiencies gained from the new system. And most IT systems are to support the objectives if the business, they aren't the actual objective if the business. A software development company can lose sight of this because it IS the objective of their business :-)