A preheated cabin and a windscreen cover are no use if the screenwash is frozen in the bottle, in the pipes and in the nozzles ??
And while an ICE will produce excess heat that was only really effective when the screenwash was mounted on the bulkhead, nowadays they’re all near the very front on the engine bay or under the wing… furthest point from the hot exhaust and most exposed to the cold.
No warning light is annoying but if you’ve got a rear wiper it’ll run out of fluid first so it gives you a little warning to top up