The state has not started large campaigns in other areas compared to electricity to phase out other fuels.
I wouldn't overrate the amount of gas that is freed by nuclear power (also do keep in mind that there is also quite a lot of heat-electricity cogeneration with gas in Germany, which further complicates matters).
If Germany only didn't shut of any nuclear power plants prematurely, then emissions could be reduced significantly since coal would probably be directly replaced. But I doubt the expansion of renewables in Germany would have happened as it did without a parallel nuclear exit in the 2000s. Would it have changed much if the exit in the 2010s didn't happen in terms of renewables? I don't know. But I also think a non-exit would have been politically untenable after Fukushima, with pretty much every party simultaneously being suddenly gung-ho in shutting down nuclear.
I think the phase-out of nuclear, could everything else have stayed the same, was a missed chance. But at the same time western countries which held much more strongly onto nuclear, especially France, are very likely seeing a huge electricity generation issue in the next decades due to very low rates of building nuclear (with huge difficulties) or renewables (not much interest) and a large amount of nuclear power plants going of the grid (https://de.wikipedia.org/wiki/Liste_der_Kernkraftwerke_in_Fr..., the list in the englisch wikipedia unfortunately doesn't include planned closure dates). These dates could (and will) be extended, but as we're seeing right now they're already getting unreliable and nearing the end-of-life.
Since it's not a drop in replacement, driving this change through might take decades and require changing building legislation etc. I'm unfamiliar what is the situation in the different European countries regarding this, but I know there are EU countries where practically nobody heats with gas.
Building a single house or building around a heating solution is a long term commitment. It might not be easy to change afterwards. This then implicitly makes assumptions about the availability and price of said heating solution.
Politics and national security concerns are inevitable around a large scale energy questions. These things don't "just happen" in a "free market". For example large scale electricity producers or gas suppliers are "too big to fail" and are actively managed by governments.
Coincidentally the first reduction came with a change of power from the SPD/Greens coalition to the CDU/FDP and the second slump when the energy and business ministry went from the SPD in the coalition to Peter Altmaier of the CDU.
In a recent debate on energy in parliament the current chancellor from the SPD (labor equivalent) responded to criticism from the CDU (conservatives), that the current government is fixing the huge mess left over by the previous governments (lead by the CDU). I'm not a big fan of Olaf Scholz (the current chancellor) in general, but rarely was there a truer statement.
Nuclear plant run times have been extended as well, by those mean green anti-nuclear environmentalists that are in the new government by the way.
After all the point of NATO was to "keep the Americans in, the Russians out, and the Germans down"
https://www.reuters.com/article/us-eu-agenda-idUSKBN0FB06B20...
It's been going on for far longer than that and pretty much since the late 1940's.
And while Brandt was eventually replaced by the more conservative (but still social democrat) Helmut Schmidt, the general strategy of mutual economic dependency was upheld, and contributed to the eventual reunion of both Germanies.
In general, back then this strategy worked so well, that even the chancelors from the conservative CDU didn't feel compelled to change it.
Even Merkel, while far more cool and restrained towards russia than her predecessor, did not really change much in that regard. Kinda expected from a conservative though...
I think what this has really demonstrated is the danger of always taking the easy solution on energy generation. Germany seems like the best example. They decided they wanted to phase out coal. Fair enough, it's by far the dirtiest hydrocarbon. Then they also decide they don't want to use nuclear power. And they don't want to frac for gas. And they don't want to invest much in LNG terminals because ultimately they don't want to burn gas either. But renewables aren't anywhere near big enough to close the circle right now, so basically by saying no to everything else, they by default grew their dependence on Russian gas.
There are no perfect sources of energy (at least not yet), so countries have to make tough choices. Hopefully that is clearer to decision makers in Germany and elsewhere now; making the easy, uncontroversial choice every time can lead you into a big jam.