Prime Garp who already fought battles with Roger, Imu and others , went doing specific training to increase his Attack power, smashing mountains to defeat Chinjao drill in direct clash. He was in prime yet he improved specific area for a specific opponent.
Same in real world where UFC fighters in his prime had to level up his wrestling skills for specific opponent , or world champion boxer who had to use tactic to develop his stamina further in order to carry out special plan against opponent.
Football player forward in his prime years . There is a situation where his team changes coach who follows different tactics so said player develops his defensive capabilites even further . Not to mention when a still in his prime changes his role developing certain skills even further.
Being in prime still leaves room for further development in certain areas.
Roger was in his prime at GV when he was 39 its not like he gained Devil fruit later that would require further development of his DF skills / powers.
Jinbei bounty was increased not his power , he simply had actual screentime to demonstrate his powers.
Brook got stronger because his devil fruit powers become active after he dies .
Blackbeard is growing in strength because he recently gained his devil fruits so controlling them takes time.
Nico Robin is 30 now so her becoming stronger after timeskip is perfectly understandable.
Same in real world where UFC fighters in his prime had to level up his wrestling skills for specific opponent , or world champion boxer who had to use tactic to develop his stamina further in order to carry out special plan against opponent.
Football player forward in his prime years . There is a situation where his team changes coach who follows different tactics so said player develops his defensive capabilites even further . Not to mention when a still in his prime changes his role developing certain skills even further.
Being in prime still leaves room for further development in certain areas.
Roger was in his prime at GV when he was 39 its not like he gained Devil fruit later that would require further development of his DF skills / powers.
Jinbei bounty was increased not his power , he simply had actual screentime to demonstrate his powers.
Brook got stronger because his devil fruit powers become active after he dies .
Blackbeard is growing in strength because he recently gained his devil fruits so controlling them takes time.
Nico Robin is 30 now so her becoming stronger after timeskip is perfectly understandable.
Comparing with real life doesn’t make sense because it’s a shonen manga.
Chinjao vs Garp looks like a massive plot hole for now.
Other comparisons like Jinbei and Robin aren’t working very well because Oda already said that Jinbei could improve more in sbs. Robin is improving despite having been running way from Imu’s lackeys her whole life.
Roger and WB and Xebec have taken decades to develop their power level because Oda needed to flesh out their history in long term in order to put flashbacks here and there. Meanwhile Goofy gets to beat Kaido in 2 years and a few months from Goa kingdom.

nah we out here Kidd bros 
So I guess u think luffy,zoro, current people will not be of that same caliber or stronger