It is called shouen formula. No time machine need
Reason, luffy will fight akainu and end be extremely difficult fight for him.
‘And we have two arcs for luffy to get stronger before facing Akainu.

Why kaido fans are like people who read shouen the first time ?
Frizea is called strongest in the universe. But Buu, and Cell > frieza

Dragon ball super have strongest fighter in the universe 7 in current manga and we has not reach EOS Z series for future stronger enemies and GT reboot series (if possible) like shadow dragon.
Enel > Blueno
 
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