Base load is not the same as back up power.

Base load is the lowest amount of power that gets used throughout the whole 24 hour period of a day, usually between 02:00 and 05:00. This usually runs hospitals, data centres and other critical infrastructure. The pick-ups during the day, peaking in the mornings, midday, and the biggest one in the evening is consumption by businesses, homes, schools, and basically everyone else.

This increase in demand draws more power from the generation side of the grid and drags the grid frequency down (50 Hz here in the UK & Europe, 60Hz in North America).

So the base load needs to increase output to accommodate these slower pick-ups to balance the frequency and if there is a sudden spike (like everyone boiling the kettle at halftime during a football match) then an quick response power system like hydro storage is used to quickly deliver power.

And when demand lowers, the grid frequency increases so you need to reduce the amount of power being generated else you'll burn out the equipment being used to transmit and distribute the power.

Now technically it is possible to balance the grid frequency using just renewables if you have enough of them, for example you just apply the breaks on wind turbines you don't need to generate power.

However, and this is the kicker, peak power generation from renewables like wind and solar won't align with the demand for the power.

And changing people's habits based on what power is available would be practically impossible. "Sorry lads, no football today the wind isn't blowing fast enough", "Sorry madam, we can't perform an MRI today, it's overcast and still and we've already used our carbon credits running the emergency coal/gas/diesel generators we have on site and we can't spare the power" etc.

A smarter grid helps balance the supply by better predicting the demand through data collection and work out which areas are consuming more power than others.

If we have enough energy storage to store excess power from renewables to be used during high points during the day then great, we can do away with base load power stations.

But all of the technologies for grid-scale power stages are still in the research and prototyping stage. And no, Lithium Ion batteries are NOT suitable for grid-scale storage because their capacity is effected too much by temperature variations and they can't be deep cycled (fully discharged and fully recharged).

So the result is we will need some form of base generation in the near term. This is why a lot of Europe has switched to Gas Generation. Because it produces much less pollutants than coal or oil burning (though only slightly less CO2) and they're much quicker to build, a year or two, than traditional nuclear power stations which take about a decade.

Hydro is good when it's available but also has some significant problems. The biggest is that it's an ecological disaster even if the reach of that disaster is far more limited. The areas upstream of the dam flood while the ones downstream are in constant danger of flooding and drought. In the worst case if the dam collapses it can wipe entire towns off the map with little or no warning. It is objectively far more dangerous and damaging to the environment than any nuclear reactor. The only upside it has is that it's effectively infinitely renewable barring massive shifts in weather patterns or geology.

All of that is of course assuming that hydro is even an option. There's a very specific set of geological and weather features that must be present, so the locations you can power with hydro power without significant transport problems are limited.

It's certainly an option, and better than coal, oil, or gas, but still generally worse than nuclear.