No, they are viable right now: they are deployed right now, and are in use right now. During times that they do not produce, other methods will continue to be used. NG will be decreasingly viable as it becomes unable to compete with local storage, transmission lines, and finally imported synthetics.

Nukes will be wholly unable to compete. They will all be mothballed, even the new ones, anywhere that government has not mandated that they be kept operating, and coerced funding for that from a captive market or taxes (as now). Such coercion will become increasingly unpopular.

It is not clear whether geothermal will be able to compete. If so, and if with new technology it becomes practical in more areas, it will be in the mix with everything else. Geothermal is very unlikely ever to become as cheap as solar, but might not be displaced by imported synthetic ammonia.

There will of course be plenty of overbuild, in places. But with the transportability of synthetic fuel and grid power, that need not be anywhere close to what you would need for a wholly independent site. Long-distance transmission lines will favor overbuild where capacity factors are higher, but falling cost will favor local overbuild.

Anhydrous ammonia stored at a few atmospheres keeps indefinitely. Hydrogen stored underground keeps indefinitely.