You virtualise, or find someone willing to maintain some ancient version of the software on modern platforms (or you pay for it). If someone wants to maintain support for legacy protocols until the heat death of the universe, they are free to do so, but again it comes with a cost that not all projects can or should bear. Someone will have to think of how the ancient protocol works on every single software update - even if technically nothing changes that's still a maintenance cost.

Also, ossified infrastructure is not a good thing. That's yet another problem we need to solve as a civilisation. Not everything new is good but some old things are genuinely inferior and should be replaced.