I can't dispute your experience, it's possible you got lucky, or the complaints I've seen were just unlucky.

I see what you mean by the side-strap, but I'm not sure how much that would actually do to help them remain usable compared to proper side-stitching, which would hold the sole on all around, instead of just in that one area in the back.

At least in this review of some Xero's, his failed pretty early, including the side-strap retainer breaking, and his second pair seem to be failing early as well. They did honor their warranty, but at least from an environmental perspective, it's not ideal to need a new pair of shoes frequently, even if it's not costing you anything.

Technically if the glue did somehow wear off, or the outer sole wore down, all you’d need to is remove it from that strap, and replace that sole with a new one and more glue. It’s probably why they offer great warranty.

Unless I'm mistaken, Xero does not appear to mention anything about repairing shoes on their website. The 5000 mile warranty just says they'll send you a new pair. In contrast, Vivobarefoot does offer a repair service (in the UK only, unfortunately), and also refurbishes worn out shoes to resell at a discount on their Revivo website, which helps reduce waste.

Also, I was wrong that Xero does not offer shoes with recycled material, after checking their website, I see that they do. Still though, I think for the money, they just seem a little suspect as far as durability. Maybe that's a quality control issue, or a specific model issue? This review of some Xero's is similar with your experience, so I'm thinking that might be the more likely answer.