To be honest, I don't know what Miele could do to improve the capacity on their dust bags.
As a fan of the brand for more than ten years, their older IntensiveClean dust bags were just as good with a pull slip at the front. They were much thinner than the higher grid bonded HyClean bags and a lot of buyers did feel crushed to having to spend a lot of money on the newer designed bags.
Although Miele may well claim extra room, it is really down to the bag chamber design. My old S571 in its day was the starter of the S500 range, a mid-size cylinder vac - the bag would take 3 months to fill and it was an old FJM dust bag which bordered out to 4.5 litres, not the 3.5 litre capacity that the newer and more compact S4 and S6 models have.
It comes down to personal experience with bag usage.
For me, SEBO wins at the end of the day for the fact that you get nearly twice as many bags with most of their vacuums. A box of SEBO dust bags can last me 1.5 years to 2 years dependent on the model and of course usage. Since SEBO brought out their synthetic disposable bags for the Felix for example, I've yet to finish the original pack I bought.
As for the Deluxe and Kombi floor tools - I don't think it would be a good idea to have them take the weight of the Felix body completely. I don't think they could handle having all that on top and they would have to be made substantially thicker to carry the weight, not just forgetting that the vacuum would fall over from the front if the floor heads were not made bigger width wise to compensate.
This is a current problem I'm finding with my new Hoover Idol stick vac.