[mmahemoff]

As others have mentioned, there are real performance gains to be won on both client and server, but does that make it a necessity? No.

Perhaps the more compelling reason you're looking for is that HTTP2 significantly affects best programming practice.

Well-established patterns of HTTP become antipatterns under HTTP2, e.g. image sprites, concatenating files, domain sharding. Most of those originated as messy hacks to get around limitations of HTTP and introduced unwanted complexity into both development and deployment of web services. Thanks to HTTP2, they are no longer needed and in fact can actually reduce performance now that the standard handles the simple case effectively.

Upgrading therefore makes for cleaner architecture as well as the obvious perf boost. The longer you hold off, the longer you have to hold off making those updates and pushing them into production. (And again, it's of course not a necessity in the same sense a security or compliance update would be. But it's another very good reason imo.)

[allendoerfer]

While these patterns are not needed anymore, most of them are not antipatterns in http/2 reducing requests still helps you by getting rid of http headers. Most of the time it is not harmful, it just does not give you the performance plus it gave you in http 1.