I think Shanks stole the Gomu Gomu no Mi because of Blackbeard.
First, why would Shanks go to such lengths to stop Blackbeard? Even 12 years ago, before Blackbeard was even a yonko?
It’s because Shanks knows that there is some sort of connection between Rocks D. Xebec and Blackbeard that makes Blackbeard himself dangerous. Shanks most likely knows how terrifying and dangerous Rocks D. Xebec was from Roger telling him.
Shanks was only an infant at the time of the God Valley incident, but I think it’s safe to assume that Roger told Shanks about Rocks. And we also know that Shanks knows Blackbeard pretty well; they both met when they were young apprentices, and they battled sometime 12+ years ago.
I think Shanks realized that Blackbeard takes inspiration from/has some relation to Rocks D. Xebec (which we know is true from the name of Blackbeard’s ship and the other parallels in their personalities). And since Shanks knows how terrifying Rocks D. Xebec was, he wants to stop Blackbeard before he can get any stronger and perhaps finish what Rocks started. We see how worried Shanks is about Blackbeard when he approaches Whitebeard (and perhaps the Gorosei?) in an attempt to convince them to stop Blackbeard themselves.
Another thing – the timelines match up perfectly. Shanks fought Blackbeard sometime 12+ years ago and got his scar on his eye. Shortly after, (12 years ago exactly), Shanks stole the Gomu-Gomu no Mi. I think Shank’s last altercation with Blackbeard was when he realized that he needs to stop him. Why else would he go seek the Gomu-Gomu no Mi immediately afterwards?
But why did Shanks seek out the Gomu-Gomu specifically to stop Blackbeard? Well, we all know how an awakened Gomu-Gomu could theoretically act as a perfect counter to the Gura-Gura (Whitebeard’s/Blackbeard’s fruit). Thus, it would be a great tool to stop Blackbeard in his tracks.
But there’s just one problem with this theory. Blackbeard did not have the Gura-Gura 12 years ago. So, it makes no sense for Shanks to have sought the Gomu-Gomu, only to counter a fruit that Blackbeard didn’t even have yet!
Unless… ROCKS D. XEBEC HAD THE GURA-GURA? And since Shanks knew Blackbeard took after Rocks, he made a pre-emptive move by getting the Gomu-Gomu as a safety precaution for when Blackbeard inevitably followed in Rock’s footsteps and achieved the Gura-Gura.
The claim that Rocks had the Gura-Gura might sound stupid, especially considering that Whitebeard was a top-performer in the Rocks pirates, but let me justify myself.
The Gura-Gura would be the perfect fruit for Rocks to achieve his goal of destroying the Celestial Dragons. On top of the Red Line, where the Dragons live, is a place that would be super susceptible to the powers of the Gura-Gura fruit. If Rocks wanted to destroy them, then it would make perfect sense for Rocks to look for and eat the Gura-Gura fruit.
It’s not impossible for Whitebeard to have been a top-dawg in the Rock’s pirates while Rocks simultaneously had the Gura-Gura. We know Whitebeard is a fucking formidable fighter even without his fruit – just look at him at Marineford. Even when he’s in the middle of dying, Whitebeard can kick ass just by himself. So it’s not impossible for Whitebeard to have been chilling in the Rocks crew without devil fruit powers, and then have eaten the Gura-Gura once Rocks was defeated by Roger and the fruit was back in circulation for anyone to grab.
So to summarize; Shanks realized how dangerous Blackbeard was because of his connection to Rocks D. Xebec. Knowing that Blackbeard would follow in his inspiration’s footsteps and obtain the Gura-Gura for himself, Shanks made a big-brain move and got the perfect counter to the Gura-Gura; the Gomu-Gomu no Mi.
“But why didn’t Shanks eat the Gomu-Gomu?” To be honest, I don’t know. For some reason, Shanks and everyone on his crew refuses to eat devil fruits. Chapter 1, Shanks says that everyone in the Red-Haired Pirates needs to be able to swim, so no devil fruit users. Why? No one knows.
Anyway, that’s why I think Shanks stole the Gomu-Gomu. This theory has a lot of holes, and I’d love to have a discussion in the comments about them.
Theory by LewdBigPepe.