Back in the early days of gas infrastructure, before wide-spread electrification, you know gas street lights and everything, the gas was produced by gasifying coal, resulting in gas that was often over 50% hydrogen, with only ~20% methane. Rest nitrogen and CO.

Natural gas has a methane content upwards of 75%, which meant that everyone had to switch out their burner nozzles but the rest of the infrastructure stayed intact.

All this is to say: Nothing about is really new or rocket science. Europe is certainly creating a backbone pipeline network for hydrogen, parts of it new pipes, other parts re-purposed natural gas pipes, many were built to a standard that allows them to carry hydrogen though some valves etc. might need upgrading. Some of those were originally built for hydrogen in the first place, and checking Wikipedia there's actually a 240km segment in the Ruhr area, built in 1938, still in operation, which always carried hydrogen. Plain steel but comparatively low-pressure so it works.

Oh and have another number: According to Fraunhofer, Germany's pipeline network can store three months of total energy usage (electricity, transportation, everything). Not in storage tanks, but just by operating the pipelines themselves at higher or lower pressure.

And we need that stuff one way or the other: Even if tomorrow ten thousand fusion plants go online that doesn't mean that the chemical industry doesn't need feedstock, or that reducing steel with electricity would make sense. Both of those things need hydrogen.

Fusion is still in the future so the plan is to import most of that hydrogen, mostly from Canada and Namibia, in tankers carrying ammonia which is way more efficient that trying to compress hydrogen also ammonia is needed for some processes anyway.