Interesting question. I hope a structural engineer or automotive engineer weighs in on this.I’m no expert but doesn’t the ‘unibody’ construction mean that they couldn’t easily have a removable roof?
My initial hypothesis is that they could've made a unibody convertible Defender, as pretty much all other convertibles for sale today (except for the Wrangler, Gladiator, and forthcoming Bronco) are unibody. The difference is that they'd probably have an exposed roll cage instead of the active rollover protection system that most convertible cars use.
But most non-convertible SUV's and crossovers have roll cages. So I don't see why they couldn't just have an exposed one instead of having it covered by body panels and sheet metal.
But I'm not an engineer and would welcome the expertise of someone who is.