This is the view I have held for a long time. As I explained in the post below curtailment of renewables can become a very serious problem at high penetrations. Obviously it varies a lot on location and renewables. Curtailment is a far bigger problem for solar in Germany than for offshore wind.
So getting to a majority share of renewables will almost certainly require lots of storage. Otherwise you have to curtail a vast amount of electricity which will just make higher penetrations incredibly expensive.
Of course some people will point to Denmark and say that they are showing you how it can be done without storage, but Denmark is a unique case. If it is really windy then they can export the electricity due to them having an incredible number of interconnectors.
You raise a good question about what penetration levels renewables can reach. Capacity factor is probably a good rough ball park at the minute, until storage comes along.
For example The National Grid has advised the UK government that there should not be any more than 60% wind/solar on the grid at any time. If this is not relaxed then obviously it places a significant limit on RE in the UK.
However an issue perhaps worth exploring, if you are going to, is looking into how different electricity mixes work with storage.
There is some interesting work out of Stanford that suggested that non-geological storage does not really work for wind, but does for solar.
It might then be interesting to see how well storage works with different nuclear/RE mixes. And it is possible that generalisations such as "nuclear works well with renewables so long as there is storage" are a bit too simplistic.