Plenty of privately owned buses were destroyed during the Troubles, same as state owned. None was insured if the incident was deemed a terrorist event. But in that situation, all were eligible for Government compensation instead. There wasn’t a “.. question of who pays for the replacement..” The Northern Ireland Office had, and still has, a department that deals with such claims. It was a routine process. (Similar arrangements applied throughout the UK during WW2 when dealing with enemy bomb damage claims. This is not new legal territory).
Plenty of private bus companies operated here all through the Troubles. There was always plenty of private hire activity, plus the many cross border bus services including, for example, the daily Gallaghers service from Donegal Town to Glasgow, via Larne. Indeed, like this Donegal to Glasgow service, many were run by companies based in the Republic of Ireland, never mind by UK based companies, nationalised or not.
Life often went on here quite normally, all through the Troubles. Many privately owned bus companies made money all through the Troubles. Basic economics tells you they wouldn’t have continued operating had the finances not made sense.
So terrorist activity wasn’t in itself a legal or financial obstacle to privatisation.