The largest 500 retailers is an excellent representation of major retailers. It's also a reasonable indicator to use for general consumer behavior, because what makes retailers the largest is their use by consumers.

You could claim that a loss of 2 retailers isn't significant proof of decline, although since it's 2 of 5, a 40% decline seems notable to me. But regardless, having less than 1% market penetration in this segment (and declining) is a pretty good sign that it is not currently successful. If you pitched some VC partners with that as proof of traction, they'd laugh you out of the room.

The BitPay post is definitely interesting, but they're cagey enough about their numbers that it's hard to tell what's actually growing. You might be able to use it to make an argument that BitPay is succeeding in some other segment. But it definitely doesn't prove mainstream consumer ecommerce success. Which apparently isn't interesting to you, but to major ecommerce companies, it's very interesting indeed.