Millbank bag? It serves a great purpose, it clears out muck and debris from water before you treat it, have you ever used one or looked at their design purpose? Perfect combined with the likes of the Sawyer filter and cuts back a lot on the need to 'back flush' you filter of muck and grit. The bag is not meant to be a purifier.
It's a water filter that doesn't actually do anything to ensure the water is safe to drink, i.e. pointless.
Back in the day they were useful as they filtered out most of the larger bits of detritus, so made either chemical or boiling a little safer (larger clumps of detritus can protect/shield certain bacteria/viruses from weak chemical or insufficient boiling).
These days water filters are smaller, quicker, lighter and actually filter the water to the point where it's safe to drink so have made the system pointless.
With regards to back flushing compared to using a Millbank bag.
Back flushing takes 1 min to do and even using dirty water it only needs doing once every 5 days or so in my experience.
It's a simple procedure using a very light weight syringe (34 grams).
You're suggesting carrying a bag that weighs around 200g dry, filtering the water through that, which takes mins rather than seconds, THEN filtering to water again to make safe.
All this effort, expense, weight and time EVERY SINGLE TIME you filter water just to save 1 min every 5 days
I think yours and my definitions of perfect are a fair bit off.
As i say years back they served a purpose, these days just use a filter, if the flow slows take 1 min to backflush it.
It's light weight, simple and effective.