Usually, parties alternate in the office in liberal democracies, but if it is the wish of the people to keep a party many years in the office, this is a democratic decision.
The social democrats ruled Sweden from 1932 to 1976. The christian democrats ruled Italy from the end of the WW2 to the early 1980s. The christian democrats ruled the FR Germany for 50 years during the 70 years of the existence of this state. The CSU ruled Bavaria for many years.
But in other situations, the same party rule a country for many years because the democracy is flawed, the elections are rigged. Like PRI, which ruled Mexico from the 1930s to 2000.
I don't know about Japan. Was the long time of the rule of the liberal democrats clean? Did Japan have a well functioning democracy? Or was it somehow a flawed democracy?
Actually, until 1993 (but they were always coalition governments so there were some some heads of government that were from other coalition parties in the 1980s)