soft Brexit would have been single market, Norway style. This was rejected by UK, not by EU

Norway style means that UK would still have had to allow freedom of movement. If the EU and especially Germany had adapted their (imo insane & ideology driven) immigration politics, I don't think the UK would have left. But in the given situation, what could UK have done?

(nothing)

immigration is the right thing to do, Germany will reap the benefit of the millions of people they have brought in. The issue is how to integrate, which is an entirely different problem. So far, no other mechanism to stymie population decline has worked

You cannot blame the EU for the UK's hard brexit. That was all down to the UK politician's brexit purity spiral. Theresa May tried to moderate it somewhat, and was swiftly thrown out because of that. This was all internal to the UK. The UK insisted on a hard brexit.

The stock "it's the EU's fault!" response is nonsense, again.

The UK wanted a hard Brexit.

There was no "soft Brexit" on the table. What you are asking for is the equivalent of changing the constitution of a country for the benefit of a single city. The slippery slope isn't that the country would fall apart, the slippery slope is that the country would abandon its core values for the benefit of a small group of elites that doesn't care about the country.