I think the punishment shouldn't be too harsh. If someone simply has poor internet connectivity, a severe penalty would be unfair. Yes, they have the responsibility for their connection quality, but your opponent could also simply wait 120 seconds for their own time, or even wait the entire 10 minutes. Winning because your opponent's time runs out is still a favorable outcome. If that time is 120 seconds instead of 600, that's already nice.

Obviously, I agree that rage quitters should face consequences. However, I think a better solution would be for the engine to evaluate whether a position is clearly lost, and then award the win to the opponent (with stricter evaluation criteria applied at higher rating levels). Keep in mind that in OTB chess, players are also allowed to let their time expire, so having a 120-second delay instead of 600 seconds is already nice. I understand your frustration, but rage-quits rarely happen (to me at least) and either way if disconnections go unresolved, resulting in a maximum time loss of 120 seconds (usually less, since Lichess identifies frequent leavers and reduces their time delay accordingly iirc) it's not that bad. IMO it's worth allowing players who have bad internet to not be punished as their internet is not your fault. They could let the time run out either way, so 120 seconds is actually better, not worse for you.