Engineering was supposed to maintain the current software, but they (engineering) chose to deprecate it and make a new version from scratch. It is just taking way longer than expected, three years and just a fraction of the original functionality.
This seems like a key issue. It's time to evaluate progress on the re-write and work out what to do next, with all options on the table (including re-focussing on the old software, or even starting afresh with a "version 3").
Are there any people in the engineering group who have been doing non-trivial bits of work on the old software? It may be worth cultivating these people and taking their opinions seriously, even if it ends up putting other (perhaps more senior) noses out of joint within the engineering group.