It is a problem that shows up at 20% - if you have enough renewables to supply a fifth of all the kwh's you consume, then when high output and low consumption coincide, 100% of your current electricity production is renewable, and all else must be shut down.

Not really - this would be true if your 20% of renewables were all from one wind farm in one location. The reality is that, as noted, you have many different renewables with different production profiles (not to mention that demand itself is not flat over the day, so "100% of average demand" is actually much less than actual demand a lot of the time) - and even within wind you have different production regimes and thus it is actually quite hard to reach 100% capacity at any time in a wind system - and the larger your sample, the harder that gets.

Now, taking into account existing hydro storage, the difference between solar and wind, the fact that offshore wind is at 40% capacity factor with a production profile closer to that of demand, and so forth, and you get a much higher threshold...

Guess what: Germany is at 20% this year, and as far as I can tell, there were no big tensions in the system...

The vision most people here have of the future grid is one in which at least 90% of all electricity production is low carbon, and supply and demand is managed via a supergrid, storage and demand managment. This is a fair description, yes?

I don't know about 90% myself, but, yes, significantly above 50% should be a target for the next 20 years (in my case, but maybe I'm a minority there, I would not mind keeping a decent chunk of nuclear as base load).
And once this is achieved, we'll have much better visibility on what needs to be done for the rest. Why make that plan today when we don't have all the info?

Nobody is seriously investing in storage, nobody is working on the problem of continental scale grid interconnection.

That's simply not true. The EU has been busily pushing for grid integration and development of pan-European networks (not just for electricity), and the wind and solar industry are the biggest promoters of initiatives like the super grid or desertec. The unbundling of networks pushed by the EU is now almost complete and one does see that independent grid operators are much less hostile to renewable than their former utility owners, who were conflicted in that respect, and grid development is certainly happening more easily when the investor the grid company) is not the main obstacle...

And one other big shoe will be moving private transport to the grid, which will have a major positive impact on the reduction of our fossil fuel consumption as well.

Wind power