Warning: Manga spoilers ahead!As Eiichiro Oda's One Piece gradually approaches its end with the manga within its final saga, recent developments have left fans wondering where the story will head next. The current Egghead Island arc has showcased Luffy's final form 'Gear 5' for the second time, after defeating Kaido at the end of the Wano arc.
While it is clear to see that Gear Five is incredibly powerful, with the Hito Hito No Mi being regarded as potentially the most powerful fruit in the series by some fans, questions still remain. Many assume there will be consequences of using the form given the impact previous forms had on Luffy, and this time around, it may be the most detrimental given how powerful it is.
5 Shortened Life
With all the previous Gears Luffy has used specifically Gear Second and Gear Fourth, many fans will be aware of the fact that these had effects of gradually shortening Luffy's life span. Given its strain on his body, these forms were only used when absolutely necessary coming up against foes who were like-for-like. In Luffy's final showdown with Kaido, there is a short moment that shows Luffy powering down and looking extremely fatigued and old in his base form.
While this could likely be a result of the long and hard-fought battle with Kaido on Onigashima and his way up to it beforehand, it could also imply that Gear Five has the biggest strain on Luffy's body out of all. This is very similar to Gear Third in its pre-time skip phase which left Luffy in a baby form after using it, but this may turn out to be more harmful.
4 Disease
Speaking of a shortened life, while the former 'Pirate King' Gol D. Roger's death came from an execution after turning himself in, this was only due to him finding out about the disease he had which would eventually lead to his demise. As it currently stands, it is made clear that Roger was not a Devil Fruit user which implies that the said disease did not come from anything related to overexerting his body for abilities, but given the huge and obvious parallels between Luffy and Roger, it would not be surprising to see the same thing happen again.
While the likelihood of this happening to Luffy is low considering the implication of Luffy being the 'one to surpass the Roger Pirates' which suggests a different ending for Luffy and the rest of the Straw Hats, it is not completely out of the equation. Fans will know from Oda's use of foreshadowing and parallels between characters usually mean a lot more than many may think, and it would not be unreasonable to imagine the Devil Fruit's properties having an everlasting effect on Luffy.
3 Shorter Endurance In Gears
As pointed out by Kaido during his fight with Kaido, Luffy's endurance was pushed to its limits and this was reasonable considering the prior rounds of their bout and his encounter with Ulti before reaching the roof. With Gear Fourth, Luffy still struggles to maintain his form and is unable to use Haki for ten minutes once powering down. With Gear Five, this is clearly already different as Luffy was able to instantly power up once more after his fatigued.
Ever since the end of Wano and the climax of Luffy against Kaido, Oda has not yet revealed or implied that there is any major downside in the aftermath of the use of Gear Five, but the chances of there being huge downsides are very much still possible. This is also very much dependent on the situation and how long the ability is used, and this was especially prevalent in the pre-time skip phase in the aftermath of Luffy's fight with Lucci and his inability to move.
2 Physical Strain In Base Form
While this more than likely would not be a factor in the current stage of the story, considering Luffy currently being within peak condition physically and it being the final saga many have been waiting for, it could very much catch up to an older Luffy. If the ending of the story is set to contain any sort of time skip to showcase the Straw Hats years later as veterans, one of the more obvious downsides would be a strain on Luffy's physicality.
There has been an obvious pattern set out by Oda in One Piece, which makes it clear that the old characters are not as strong as they once were, such as Garp and Rayleigh. If Luffy is far more active in his use of Gears which are said to shorten his lifespan, there is no telling what impact this could have on his body in years to come.
1 Physical Appearance Altered
Gear Five gives Luffy the ability to fight freely and without much constraint as shown in his bout with Kaido and now Rob Lucci. This means Luffy can be hit with deadly attacks that now leads to his physical appearance forming either a shape of the weapon, or showcases the manner in which he was hit.
While that is strictly down to the properties of the Hito Hito No Mi Model Nika fruit and Luffy's capabilities of shaping the environment to his liking, perhaps as a result could be Luffy appearing much older than he is, which could relate to why he looked incredibly drained and more like an old man after powering down against Kaido.