What spec of DOCSIS was your old modem? If it was 1.0, 1.1, or 2.0, sorry you lose all support, the older specs had hard bonded channels that HD TV on them after the swap that they informed people of for 2 years before it happened. And they put TV on them since they were degrading channels due to overuse across the entire network (as in across the country).

The later specs allowed for floating channels based on channel maps, which allowed Comcast to bypass those degraded channels.

Note: I'm not an apologist, but I worked for Comcast and for a subcontractor. Comcast treated (at least in my opinion) their customers like wallets that called and complained, but under the subcontractor I saw that since they didn't rewire 100% of all networks purchased, it was common that the older lines were causing the degradation and also reflection on other RF channels sometimes on the other side of an area even. Now if Comcast invested in their network as opposed to buying other companies and calling it investment, this might have been fixed, but that would be decades vs. having every modem that wasn't compliant to the new spec swapped.

The SB6121 is a DOCSIS 3.0 4x4 modem rated for 174mbps, SB6141 is a 8x4 rated for 343mbps, and SB6181 is a 16x4 rated for 686mbps. Outside of their capabilities, the hardware on them are nearly identical. There is nothing "EOL" about the SB6121 except for the idea that it's unable to support 200mbps. It's a perfectly good entry-level modem capable of offering speeds that are over 7 times the minimum definition of "high speed internet".

I don't understand the general attitude against forced modem upgrades. If you lease your modem it's as easy as walking into a Comcast store and swapping it for a new one. If you own your modem, pick the newest model of modem that fits your needs.

The newer modems support more channels and newer modulation/technology. This isn't just about supporting newer speeds. In order for them to support those newer speeds for other customers they have to upgrade their equipment to support more channels and newer modulation/technologies.

At some point these older technologies are not just wasting resources by being less efficient, but are preventing the company from upgrading their equipment.

The reason I don't understand, is because it's common to see people complaining about the state of broadband in America compared to other countries. Yet Comcast is probably the most progressive as far as pushing the technology goes. Don't misunderstand me, I believe Comcast holds a near/total monopoly in many locations around America but at least they're progressive with their network and technology despite the lack of meaningful competition.

  it's as easy as walking into a Comcast store and swapping it

If you have an Xfinity store nearby, and if they don't have lines over an hour long much of the day, and if you get a rep who knows what s/he is doing.

I live about 6 minutes from one, and it can still be a multi-hour adventure.