Numatic Henry was originally designed for commercial use, and as such lacks the refinements of the miele models, such as wheels on the floorhead, and storage on board for the tools. Most companies only vacuum hard floors or short pile carpets with a Henry, and its metal baseplate is hardwearing and cheap to replace.
Filtration wasn't really an issue for companies, and most Henrys are still used with cheap paper bags or bagless (those where I work are anyway)
The Henry was never designed to be a direct competitor to the Miele, which was always designated a domestic vac, and always cost substantially more than the Henry.
The Miele does have better suction power than the Henry, but then the Miele has a much more powerful 2000 - 2200 W motor, whereas Henry has only a 1200W, and for its power rating, Henry still pulls a fair amount of suction.
Hepaflo bags are not better than Hyclean bags either, but an improvement on the old paper bags Henry used to use, and a fair bit cheaper than Hyclean bags.
Many people prefer the Henry as it is tried and tested, cheap and basic, and also cheap to buy spares and bags for.
I have both a Henry and a Miele S6, and Henry is used for doing the car and the dirtier jobs that I don't want to damage the Miele with. As far as I'm concerned, the Henry will always be a commercial vac, and its a very good commercial vac, withstanding rough treatment that would ruin a Miele very quickly - for example say, in the back of a builder's or plasterer's van.
Also, I have never noticed any dust build up in the Miele S6 and have always used Hyclean bags - I think they are expensive, but work very efficiently.
My Henry has always used the Hepaflo bags, and his tub stays very clean with them too, so if the vac is maintained and looked after there is no reason why the tub or interior should get caked with dust.