A pair from 2015 that's still holding up is definitely solid stuff. Converse are built to last - they age well if you take care of them. As for the cracks on the sides, it really depends on the overall condition. If it's just cosmetic and they're still comfortable, honestly I'd keep them. Getting them repaired often costs almost as much as a new pair, so it's something to weigh. But if the cracks are starting to turn into holes or the sole is peeling off, then it's worth finding a cobbler.
As for wearing Converse in 2026, don't worry about it at all. They're a classic, they never really go out of style. They're everywhere, from teenagers to adults. The worn-in look you're describing is actually pretty cool - it feels more authentic than brand-new and pristine. What would actually be lame is wearing something just because you think it's trendy. If you love them and you've been wearing them for 11 years, that means they're your thing and that's what matters.