Overprovisioning by how much? Currently ( at least since a month or so) across Europe there's a meteorological phenomenon that makes for cold temperatures, lower than usual wind speeds, and clouds. 2 out of the 3 main renewable power sources ( solar, wind, tidal) are heavily impacted ( wind most of all iirc) and producing less electricity than usual, while people need to warm themselves. And this is expected to continue into the winter.
To combat such a scenario if everyone was on renewables+storage+interconnects, you'd have to built massive storage ( which we don't have the tech/space for) or massive interconnects from really far away places like Africa.
Not if you take into account land use. A single nuclear plant can supply about a GW of electrical power. The equivalent capacity in wind and solar--even without taking into account the overprovisioning that, as you note, is required because of the lack of reliability of those sources--takes up far more land than that nuclear plant. That has a huge environmental impact that nobody takes into account when figuring the actual cost of wind and solar--though of course the same people are quite happy to hunt down every last little environmental impact of nuclear and point out how it increases the actual cost.
(The requirement for massive amounts of storage, which others have pointed out, is another environmental impact of wind and solar that nobody takes into account when figuring the actual cost.)
However, the new reality is that wind/solar are cheaper than the old baseline plants. Now wind/solar do not supply us a with a constant power like those old baseline plants, but instead with variable supply as well. So we still need fast adjustable sources (or storage), which now even out both variable supply and demand. By over provisioning (and net interconnectivity) we can reduce the need for adjustable sources. Essentially if you have enough renewable sources to cover the high demands at low wind/sun you can always turn off the sources when you overshoot and because they are cheap it is economical unfeasible.