The process was quite fast mainly because most Finns are very pragmatic, clearheaded people and intuitively understood how the rules of the game had changed. You can't game a 76% support for any cause in a few months in a liberal democracy.

The root of the matter is that without NATO membership conscription would have become morally untenable. And hence the backbone of Finnish defence would have collapsed leading to need to kowtow to Moscow for all eternity.

Before invasion of Ukraine the economic costs on top of Finnish defence forces were considered to be sufficient deterrent to Russian agression, on top of the material and personnel costs such an invasion would cause.

Ukraine demonstrated that with political impetus, Russia does not care about either economic costs, nor about materiel or personnel costs in it's mad imperialist fantasies.

IMO The cost of NOT joining NATO would have been de facto loss of sovereignty.

Furthermore, due to the closeness of the Russian border, without NATO membership Finland would likely have been perceived as a not very good target for capital investments, killing the economy. While Finland has advanced technology, it is very capital poor - hence needs healthy injections of capital from the global market.