"people still use this voucher biz – who knew?"
What surprises me more is that Groupon didn't die to to a lack of new *businesses* signing on. It became clear years ago that Groupon had hugely oversold many the idea that all those people they'd get through the door with (in effect) loss-leaders would become upsellable repeat customers, when the reality was that most were only interested in getting as much as they could as cheaply as possible from them before moving on to the next Groupon.
Having had their business disrupted by Groupon shoving way more customers than they could handle through their door (another problem IIRC), disrupting their regular- and profitable- business and clientele with nothing to show for it but way more losses than expected and Groupon adding insult to injury by taking their cut... well, you'd assume that most who hadn't already been burned would have learned from others.
From what I remember, it was generally considered that the only businesses Groupon was considered a sensible idea for- if any- were those who ran services, classes and so on that had larger or more malleable profit margins and where a significant proportion of buyers were likely to end up not using the vouchers they'd purchased. Or those with a large amount of some arbitrary thing that they needed to shift but could afford to discount significantly.