Ever see a Shidokan tournament? Don't know if they really do that anymore but it was like a hybrid fighting event where fighters (mainly standup guys--karate, TKD, western kickboxing and maybe some Muay Thai) would fight three rounds, but different rules per round
Round 1 - similar to Kyokushin Karate rules--no gloves, no punches to the face, full contact
Round 2 - kickboxing rules with 12-16oz gloves
Round 3 - "No Holds Barred"--basically MMA rules with 6oz gloves, but VERY limited as you only get like 30 seconds to work on the ground
The primarily standup fighters almost NEVER tried taking the fight to the ground in the 3rd round. It was basically a kickboxing match with 6oz gloves.
Similarly I highly doubt a boxer who's been throwing punches their whole life, will suddenly start grappling to finish a fight.