I agree with you on the mayhem modifiers being flawed and the difficulty being too low (other than circle of slaughters) but honestly I think multiple playthroughs should be entirely optional.UVHM requiring slag is mostly a footnote in that context, though; the eHP changes in enemies were balanced around it, and BL3 doesn't have the mechanic to balance around anymore, so it isn't really a concern. I'm not against using eHP as a function of increased difficulty - I actually think the base game probably didn't ship with a high enough ceiling in this regard, even on Mayhem 3 - but I just don't think Mayhem as a mechanic is the thing they should be using to drive that function of difficulty scaling when we already have an existing one that already serves that purpose. I suppose on some level they were hesitant to split the game into 3 full campaign runs to avoid the issue of players not getting enough skill points in the first playthrough (the only one the majority of players actually experience), but I think anyone who does decide to go through the TVHM playthrough would have been interested in the UVHM playthrough that scaled enemies up to at least the level of the current Mayhem 3 average.
One thing I think is pretty certain however is that we definitely do not need Mayhem modifiers acting as positive force multipliers on player damage. That feels like one of the biggest mistakes they could have made with the system, and is the thing that ensures Mayhem never feels "good" by multiplying the feeling of negative impact of "bad" modifiers. There is currently absolutely no incentive to endure a particularly punishing set of Mayhem modifiers when simply quitting to the main menu and reloading can result in the player becoming vastly more effective instead, without any downside in terms of reward. As designed the mechanic is going to encourage player behavior they certainly did not intend with the Mayhem setting, while simultaneously watering down what difficulty exists in the base content, which I'd say already is not at the level it needed to be for the current level of gear scaling they shipped with.
This is again something they could have aped from Diablo 3, had the mayhem levels just increase enemy hp/shields and loot/currency/xp while the mayhem modifiers are just applied to groups of mobs or bosses or what have you.