In every company, we have situations where some legacy products. Legacy can be in sense of programming language OR platform OR UI OR features OR anything else you think 'legacy'! But still we need to work on it because we have clients using it and we need to provide maintainability/support for them.
Only cash can not make the developers happy. You should provide them good environment so that they can pay attention to their work.
Another thing is no technology is bad OR legacy OR older. Thing is if your company needs to maintain it, then you must keep it going. But keep all standards for designing, coding, testing, code review, interactive sessions etc...
Also you can motivate them to convert your legacy code to some new platform for better performance and maintainability. Every company does that even once. Why?I think, because they want to compete with other market products.
Also provide them some cool sessions for other technologies which are used in your company but they don't know or use. Let them be deeply in the things, give them proper time and support and be problem solving. Main goal is to deliver on time with less rework and bugs.
Provide some rewards towards their work and keep them happy about their work. After all work is what we do together. If the team feels right, they will do right and develop right.
Until your product works fine without any problem and client is happy then you can think to add new feature to it. Today technology is better and we can integrate different applications without any programming language bar. You can do new development in new-age languages and integrate them with your product, which will keep your product ready for today's competition.
