I've got the Royal Enfield panniers and they are brilliant. Solid. Well made. Waterproof in heavy rain (proven). Lockable. I love mine and they're on the bike constantly. I love that I can stop on the way home and do some shopping.
However, because they're on the bike constantly, I carry too much stuff - full water proofs, three pairs of gloves (summer, intermediate, winter... and there have been times where I've happily changed from one to another during a ride), a hat (coz I'se bald and have skin cancers on my pate), two shopping bags (filled carefully, one will go each side with room for more and that's over a week's shopping for this single bloke), disc lock, cable lock so I can leave my helmet on the bike while shopping, binoculars (seriously).
They're riveted together, as you commented, and these interfere with removing the panniers. If you're going to be taking them on and off, you need to back off the bolts for the lower mounts slightly (maybe a turn) to allow a bit of flex in that mount. If those bolts are done up tight, the rivets foul on the frame. I don't remove my panniers, so I keep them tight, but I don't seriously think it'd be a problem if you kept them loosened so you could remove the panniers. In simpler terms, it's not perfect but it's not a problem.
The frames for the panniers are an enclosed rectangle. If I ever go touring on dirt, I'll use my soft panniers - firstly because they hold more, secondly because it's more fun being trapped under a soft pannier than a hard one. My soft panniers have a strap on each corner and I will be able to strap the panniers securely to the frames while having them held in place away from the bike.
The panniers are very strong. I've whacked them against things and dropped it on its side once (mucked up moving off didn't I) and they're essentially unmarked. Very well made.
The Royal Enfield panniers are well worth considering and a brilliant product for my use.
There's an Australian company (Queensland I think) that makes panniers out of the same plastic as wheelie bins. Their promo video shows them driving over a set with a four wheel drive, then massaging them (with heat) back to their normal shape. I don't have a link but might be able to find it if you're interested.
Then there's this option which I know nothing about because I only found it while searching for the product mentioned above
https://www.motorcycleadventure.com.au/parts-finder/royal-enfield/himalaya/