I could see that, if I was ever to have that many colors, which I doubt will ever happen. I’ve got black, white, red, orange,
blue, yellow, and wood. I may add some green. That’d be about it.
One thing about the bag system: how do I splice ends of rolls together? That is still something I want to do.
A two part printed spool will separate everytime you want to use a specific refill filament. Open the bag, pull out the filament, put on the spool, print, take it out of the spool, put it in the bag, close the bag. And so on!
A simple piece of tape isn’t going to close that bag. What you describe is basically what Richard Horne (RichRep) described on YT. Don’t get me wrong, what @Geit described is quite interesting. I can even see a modification wherein a small valve is added to the bag, so the filament can be fed out through it, and then pushed back into the bag and a hinged cap closed to seal the bag again.
I didn’t say anything about " a single piece of tape". Sealing a bag to prevent the moisture to get in, is another thing!
I understand. What I was trying to do is reference @Geit’s comment about how a simple piece of tape could be used to reseal the bag. Taking the filament out of the bag obviously makes that impossible, But I do get your point: a master spool concept could be used to only hold filament when it is actually being used. The rest of the time the filament stays in the bag. It would certainly greatly reduce the number of master spools needed, maybe to as few as 2.
It would be an interesting blend of the 2 approaches. Resealing the bag is, as you say, another thing. It would have to be solved. Unfortunately, ZipLock bags just aren’t airtight. We need bags that are airtight.
Is bagged filament cheaper than filament with a reel?
In theory, yes, but I think you will that, at least in North America, your choice for non-spooled filament is very limited.
It would not be worth changing your system then.
Yes, blending two approaches would made the best result. Just think of Geit’s filament dryer that could be blend here two (!!!)
Well, unless my local filament store isn´t selling any spools without an actual spool I stick with it. Maybe they take the spools back and have a way to recycle then for new spools.
I really have to ask them the next time I visit them. The empty spools here are piling up again. 
If you have a local retailer that sells filament in bags then you are really lucky.
It is already great to have a locale reseller. This saves tons of money for shipping and the opportunity to just decide to get a special filament or a new spool right now is amazing.
I have a local retailer but his selection is somewhat limited. Fortunately, I have Amazon Prime, so I can get filament with free shipping.
Unfortunately this is not always true. If you compare prices on Amazon with the prices of the manufactures itself there is usually a difference about 5€. That’s the Amazon tax. I checked that with some German filament brand and while the filament on Amazon was 22€ at the time, getting it from the company for less. In fact the saving of Amazon prime gets added to their pricing, so you pay basically the same, when ordering filament and paying pp. I’m some cases even a little less.
I strongly suggest to check the companies product websites which are not related to Amazon.
I have done extensive comparison shopping: Amazon is as cheap, or cheaper for me than the other dealers, especially when shipping is taken into account. On occasion, other dealers will have a sale that offers filament at a slight saving, but it’s almost always only for PLA, not the PETG I normally use. When I’ve used up my current stock of PLA, I don’t really plan to order any more.
That has been my conclusion too. I often look around. Amazon Prime works well for me and I feel like I have more recourse if there is a problem which I had recently. I had some clear filament that was so brittle it would just snap in the middle of a print. I still haven’t yet received another. But I will get on their back.