FlashPlayer support will end soon, what now?

It's time to begin migrating your content and customers to an HTML/JavaScript solution.

While you could, and may necessarily, pursue strategies to freeze your clients' infrastructure in time (running old browsers, etc.), the browsers themselves will ultimately drop support for plug-ins and significantly increase the amount of friction involved in running Flash Player over the coming years.

There are certainly well-established precedents for running legacy enterprise applications after mainstream support has moved on.  Options range from running outdated browsers, to creating dedicated, isolated infrastructure (e.g. Citrix clients on modern machines, connecting to virtual machines that talk only to your Intranet, and maintain a static os/software profile).  That's probably not a great recipe for customer satisfaction, and may present an advantage to competitors that offer a native HTML/JavaScript experience.

Internet Explorer has historically been the choice for Enterprise applications that need to stay insulated from change, and based on what I know at this point, I see that trend continuing.  The major U.S. browser vendors have all published public roadmaps over the last year, which include the restrictions that they intend to impose on Flash Player.  Your customers will begin experiencing significant friction well before the 2020 milestone. 

It's hard to make generic recommendations for a large software product; however, I think the big question that you'll need to answer as an engineering team, is "Can we take a phased approach that starts with surgically replacing just the front end?".  Where your application is powered by backend services, can you find JavaScript libraries that let you continue to use them unchanged?  Is it more efficient to simply write libraries or simple middleware services, v.s. replacing the front and back end services simultaneously?

We understand that these kinds of changes are not free or easy, which is why we've provided such a long timeframe for advanced notification.  Depending on the scope of work involved, you may need to reprioritize engineering work; however, it's an investment in the long-term viability and competitiveness of your product.

Personally, I think you're best served by prioritizing a migration to the Web Platform now, before your customers really start experiencing internal support pain as browsers tighten restrictions on Flash Player.