Ask HN: Need help conveying concept of technical debt to CTO

My company maintains a legacy application in the primary healthcare domain written in classic asp. The app has all of the problems that you would expect with anything written in classic asp - no separation of concerns, innumerable redundancies, no tests of any kind, brittle business logic etc. Working with the system is unnecessarily difficult and can make what should be easy tasks very laborious. We are frequently turning away new contracts simply because we don't have the resource to serve new customers on the platform.

I was hired over a year ago with the explicit purpose of helping the company transition to a new platform (I've been advocating something on the JVM, probably Scala and the use of *MQ to help decouple the system). Upon starting however I was immediately seconded to a new development project on the legacy system instead.

The business is now embarking on the development of a new service, which the CTO is intent on building upon the existing platform. He's aware that the existing platform has a number of problems, but due to time constraints he doesn't foresee us having the resource to develop against a contemporary platform. I've tried to convey that doing this will further incur technical debt and even further decrease the agility of the company. I personally feel like this decision is going to hurt the long term viability of the company, but I don't think I have adequately conveyed this. The existing system continues to function, but only through the titanic efforts of our development team, and I can't help feel that because it is functioning there must be some perception that it is not broken.