I don't question that there are lots of applications well suited for browser-based apps these days. I have been pushing that as a solution since installing a Siebel application some years ago and being impressed with their pre-Ajax solution. But, one can never make these decisions in a vacuum. Not every application is sensibly accessed from a smart phone or even remotely. Do you want your stock broker managing your investment account from a smart phone?
More importantly, in the context of this thread ... we are, after all, trying to help someone who asked a question here ... we have a company who has been too cheap to keep up the maintenance on their existing Progress application. Moreover, the little we know about the application and architecture, it sounds like it is probably fat client, no AppServer, no ESB, no layering. So, rewriting it to be browser based is not only pretty close to a total transformation of the application, but is going to require additional licenses when they are already spooked by the cost of rebuying the Progress license.
So, choice are:
1. Dump the current application and buy some other COTS application.
2. Dump the current application and build a new application using a new technology.
3. Relicense Progress and completely rewrite the application to make it modern.
4. Relicentse Progress and make gradual improvements.
5. Do nothing.
Assume #5 is not an option. #1 is very expensive in licenses and loses all company specific modifications that help run the business as well as limiting the changes that can be made going forward. #2 could be anything from free to moderate in license costs, but is going to take a huge investment in programmer time, so it is going to end up being expensive unless the current application is trivial. #4 is moderate in license costs, but could be expensive in developer time depending on how much modernization one wanted to do. It could be justified, but only if there is a good business reason and those could justify #2 as well. #5 is modest in license costs and requires almost zero developer time unless or until one decides to make some incremental change. I.e., it allows going forward indefinitely, preserving all existing custom business logic, and allows one to make spot improvements on a project by project basis as they are justified and affordable.