Some/most of the champions that are at the bottom of the winrate list are just bad teamfighters. Evelynn, Elise, Lee Sin, Kha Zix, Zed, Akali; all champions that are poor teamfighters because they were designed in a way that made them strong as assassins, they do best against isolated targets and in ARAM you are in a teamfight for the majority of the game. In addition, some of the lowest winrate champions in the game regardless of map or gamemode make up a large portion of this pool.
If you notice, champions that did not have very high winrates (ziggs level) until the addition of dash/mark were strong teamfighters, but are now able to engage better and stick to targets. Of those I mention, Sion, Nautilus, Maokai, Vladimir, Galio, and Mordekaiser were almost never top 15 champions for ARAM prior to the addition of Mark/Dash (Galio might have been). You'll also notice that Vel'koz was dethroned as one of the strongest ARAM champions. Janna and Varus have been dethroned as well.
Basically your argument that bad aram champions are still bad at aram is true only because Aram is a teamfight/siege start to finish. Champions designed to do other things in Summoners Rift will be bad in an element of the game they are weak in. Not to mention that some champs are just bad right now. Bard is a champion that is not designed for the Howling Abyss and it shows. His E is almost useless in the map. His chime spawn locations are hard to place in such a crowded space. His only saving grace is his Ult and that takes coordination with a team that is not found in a random team.
There are some new stronger ARAM champions though because they have a more solid way to engage on champions they previously had trouble with. I almost exclusively play ARAM and I'm one of the few diehards that welcomes this change and even though mark/dash is kind of unbalanced at the moment. It is a good addition to the game mode. I have a slightly positive win rate on ARAM both before and after the addition of Mark/Dash with over 1300 games played. My champ roster:

My most recent purchases have been Poppy and Nunu. Champions that have been gifted to me: Thresh and Malphite
I'm sharing this information to show that I might have a bias based on my experience whilst playing ARAM. I play to have fun; even though I try to win every game, I don't play with the sole purpose of winning (probably why I don't play ranked). If you do, hey, that's you; I'm not one to judge. I think the Mark/Dash summoner adds a new element to the game, but I do think that it is a tad OP right now, and I'm glad that there are people working at Riot to bring the spell to a healthier spot in the game mode. It doesn't really need much, they've already announced splitting the damage, which is good. The only thing it would really need is a slightly higher CD and possibly reducing the range a tad. I would say a 30 second CD and a reduction in range of about 500-1000. With those changes and the split damage, I think it will be fine. After that, I hope everyone can be happy.
Some champions will still be weak in the Howling Abyss, and that is okay. Some champions aren't meant to do well at teamfighting and sieging.
to name a few. And some champions are not well suited for the Howling Abyss map:
and
namely. And still more, are really weak right now due to being nerfed:
.
Bottom line is if you are playing the game for your W/L ratio and your KDA, you might not like the change because you have put in the time to understand the game as it has been for years upon years. When something new threatens the balance of that, you might get upset. If, however, you play the game to have fun and improve your own game, you probably have no qualms with the Mark/Dash summoner in ARAM. I personally love ARAM because it teaches you to adapt to all kinds of possible teamcomps; how to play with them and against them. I welcomed the new summoner as it was something new to adjust to and adapt: evolve. I think I might get Kha Zix as one of my next champs. :P
Anyway. Later dude. Good luck with everything. Peace.