I think your sentence is quite clear. Let me quote you again. You wrote:

"... when a developer is maintaining legacy code instead of helping to roll out a new project, a new revenue stream is delayed, which is lost revenue."

I think I understand now that what you wanted to say instead is "lost opportunity to potentially create new revenues." But definitely, not creating a new product because you are doing maintenance work is a lost revenue.

It's like manning a store. You have to be there every day to make sure that customers can do their business thus generating revenue for you. If you wanted to create a new store somewhere and you have to abandon the existing one then you'd definitely lose revenue from that store. The new store you plan to create is not yet a guaranteed revenue earner. It's only a potential revenue earner. Like a new product, you don't know that it could be a flop.

Developing a new product is like R&D which Wikipedia defines it as a cost center. It's not a source of revenue stream. http://en.wikipedia.org/wiki/Cost_centre