The maximum download speed for DOCSIS 3.0 is 1216 Mbps, and that's only if you use 32 channels. If you were selling 200/6 connections to 72 users then you would be only guaranteeing them each ~8% of the download capacity (and yet 50% of the upload capacity, using 8 upstream channels).
To have the same level of oversubscription for upload as download, you would give them all 35Mbps upload with the 200Mbps download. But you don't even have to do that, because who says they all need the same plan? Sell a quarter of them 100Mbps up connections and the others still get 17Mbps up.
No way to justify why it should only be 6 up if it can be 200 down. (And if they're offering 200Mbps down without that much oversubscription by using DOCSIS 3.1 or 4.0, even less excuse for 6Mbps up.)
There is only 37 MHz available for upstream, from 5 MHz to 42 MHz (basic upstream frequncy allocation). There is 890 MHz avilable for downstream, from 112 MHz to 1002 MHz.
Therefore, given that there is 24x more bandwidth in the downstream direction compared to upstream, it would make sense to offer 20:1 or higher ratios of DS:US speed to customers. Of course, if your HFC network is "good", i.e. has less users, you can sell higher speeds, or just grossly over-sell the service and "hope" not everyone uses the upload at the same time.
Also, a correction: any given cable modem can support up to 32 bonded downstream channels for ~1200 Mbps per CPE, while the cable itself has capacity for ~130 channels, which is theoretically ~4900 Mbps of usable capacity, if there are no TV channels on the cable. However, there is still about ~200 Mbps of upload bandwidth on the cable for all customers, which is limited by the number of channels in the return path RF range. You can assign multiple customers to a single channel, but you can also segment the channels so that all customers get some channels out of the available range, but not exactly the same ones, which would increase the overall capacity of the network.
> Therefore, given that there is 24x more bandwidth in the downstream direction compared to upstream, it would make sense to offer 20:1 or higher ratios of DS:US speed to customers.
And yet 200/6 is >33:1, much less 1000/6 at 167:1.
Moreover, to even get to 24x you're using the full theoretical allocation for downstream but not for upstream, which would be 40MHz from 5-85MHz (extended frequency allocation) according to your link. Which would then be ~11x, implying (on average) 200/18 and 1000/90.
Also, who decided it should be impossible to allocate more channels to upload? I realize that's more a question of "who designed this crap" than "why aren't they doing this now" but when the parties responsible are still Comcast et al, that makes it hard to want to give them a pass for it. (Though credit where credit is due, DOCSIS 4.0 supports symmetrical uploads.)
To have the same level of oversubscription for upload as download, you would give them all 35Mbps upload with the 200Mbps download. But you don't even have to do that, because who says they all need the same plan? Sell a quarter of them 100Mbps up connections and the others still get 17Mbps up.
No way to justify why it should only be 6 up if it can be 200 down. (And if they're offering 200Mbps down without that much oversubscription by using DOCSIS 3.1 or 4.0, even less excuse for 6Mbps up.)