There's a cost to using outdated systems, and this is it. If you need it, you can proxy requests to upgrade their protocol to something newer browsers speak, upgrade the device in question, keep old versions of browsers around for those single purposes (and probably gap that device as much as possible as it's now a much easier target for attackers).

Calling for all software to support all outdated options and code paths forever or even to always keep the code and flags or settings to enable it is absurd, and leads to unsustainable software that becomes impossible to change and improve.

There's heaps of old modems that use a weak DH key and will never see a firmware update. You're left with either accessing the device insecurely over HTTP, hoping your ISP will send you a new one (good luck with that) or paying for your own modem which will probably never be allowed on the ISPs network.

Weak DH keys should not be that hard to keep in the code base yet still most browsers will present an impassable TLS error screen.

Perpetuating it won't do, and if in doing so we're perpetuating a larger impending security issue, then we need to resolve it stat, not defer everything because there is heaps of old hardware lying around.

That may be easy to say and harder to resolve, but there comes a time when problems need to be resolved. Maybe that won't be 2020, if the desired timeline proves unrealistic, but two years is plenty of time to move on it. It generally takes far longer to deprecate and remove protocols from the web than it does to get a replacement modem.