It sounds like it’s a design fault in the sense that their manufacturing tolerances are not tight enough to avoid having the rear tyre hit the seat tube on some examples – surely it can’t be happening with every Following they’ve sold, or we’d have heard about it loads already?
Bikes are designed and manufactured by humans. Humans make mistakes. That’s why bikes come with a warranty. The manufacturer has to accept that some of their profit has to be spent on replacing frames with manufacturing faults, otherwise few purchasers would risk spending so much money on a bike which may turn out to be unusable.
Occasionally products with manufacturing faults manage to sneak past my business’s QC and test processes. When that happens we replace/repair them as required. It’s just one of those costs of doing business. You can try to avoid supporting warranty claims but it’s terrible for your brand because the word will get out, and such behaviour will cost you more in the long run (though if you’ve released a really disastrously problematic product – like that Evil from a few years back – you may not have the cashflow to do the right thing…)
Consumer law is on your side. Go back to the retailer and don’t back down. So what if Following’s are sought after secondhand? How sought after are faulty ones?! Appalling!