The desire for Test rugby was a major factor for staying despite falling down the pecking order under previous boss Warren Gatland, who had not selected him since the 2023 World Cup.
Asked if there was a temptation to leave due to Wales frustrations, Williams replied: "Yeah, there was definitely that, but at the same time I leant back on my reasons why and that was still international rugby.
"From the age of six my dad took me to Principality Stadium and we had a debenture there.
"My reason why was to be on that pitch in front of 74,000 fans, so I kept going and wanted to give it one more shot. Luckily I did."
Williams, who started alongside Cardiff's Ben Thomas in Japan, faces stiff competition at club level in the shape of new signing Joe Hawkins, Joe Roberts and Macs Page.
With Scarlets' Eddie James out injured, other midfield options for the autumn are Gloucester's Max Llewellyn, Saracens' Nick Tompkins and Ospreys' Kieran Williams.
"There is midfield competition at Scarlets and it's amplified at international level," said Johnny Williams.
"No spots are guaranteed and you always have to be fit, ready and fighting for a spot here, let alone at international level."