There's a non-zero risk of it going off in the ocean, which could cause ash fallout, depending on the magnitude of the blast.
Property and infrastructure damage is not the only outcome here. We could end up with ash fallout and flight disruptions. There's also a major power station in the immediate risk zone, which is a major loss if it gets destroyed.
If and when this blows up, it's going to be big.
The question is just if it's big and causes fallout, or just big. And when we're talking about property damage, it's less damage and more "will the town be obliterated or not". The magma is most shallow under the town, and there's a 15 kilometer magma pipe extending to the ocean. So the possibility of the eruption happening in Grindavik itself is also very real.
2010 showed what kinds of impacts can occur when a volcano erupts in Iceland. [1]
> In response to concerns that volcanic ash ejected during the 2010 eruptions of Eyjafjallajökull in Iceland would damage aircraft engines, the controlled airspace of many European countries was closed to instrument flight rules traffic, resulting in what at the time was the largest air-traffic shut-down since World War II.
> On 16 April 2010, 16,000 of Europe's usual 28,000 daily scheduled passenger flights were cancelled and on the following day 16,000 of the usual 22,000 flights were cancelled. By 21 April 95,000 flights had been cancelled.
> IATA stated that the total loss for the airline industry was around US$1.7 billion (£1.1 billion, €1.3 billion).
Eyjafjallajökull was a special case, it was not an oceanic blast, but a blast under a glacier. Fjallajökull means "glacier mountain". The blast flash-melted several tones of ice, which caused the lava to cool rapidly forming the ash cloud. If my memory serves, the glacier was in the caldera itself.
But the effect would be very similar. The water would vaporize and create an aerosol of ash.
I can tell you for sure there are no such heated roads in Grindavík. The one in Reykjavík is on a street that cars can drive down, but in practice don't very often. Large plows would struggle there and the pushed off snow would clog the walking areas in a very popular tourism location.
> When the volcano blows, it should only be property, building, and road damage.
People had the option to get their animals, but not everyone did so. If the eruption destroys the town some animals will be among the casualties, if they haven't starved etc. already.
When the volcano blows, it should only be property, building, and road damage.