I agree to some extent with this, one of the main issues with enchanters isn't simply enchanters, but the enchanter-marksman combo, which is exceedingly reliable and powerful. This is also why I dislike arguments that enchanters are the most powerful class in the game, and so need a nerf, because their power is being almost exclusively transferred to the marksman, which is why the enchanter subclass itself comes across as strong. Nerfing enchanters I feel would simply leave them in a bad state overall, and limit their ability to be effective outside of working with the marksman when the meta inevitably shifts.
With that said, I don't believe one class's faults should excuse the other's, and I think part of the issue here is that both marksmen and enchanters are individually too reliable, beyond just how they interact with each other. On one hand, marksmen are notorious for having damage that is essentially uncounterable and costless, on top of being very high, and on the other enchanters have a common problem of also lacking counterplay to their utility or CC, which has also harmed their prospects in solo lanes, where their core playstyle is usually revealed to actually be quite unhealthy. Janna and Lulu are the most egregious example of this, with their long-duration shields and stat boosts on top of easy hard CC, but Sona and Soraka also offer very few opportunities to counter their abilities, even if they themselves are rather squishy.
Those who have read my posts on this kind of issue before know my opinion already, but I think the core problem with enchanter design is that it's predicated on an incomplete notion of counterplay: Jinxylord mentioned this a few weeks ago, but one of the main obstacles to adding counterplay to enchanters right now is that the most common tools for counterplay, e.g. delays, skillshots, things that can be anticipated, don't really work on supports, whose utility generally needs to be on-demand and reliably applied if it is to work in sync with teammates. Heroes of the Storm somewhat challenges this assumption with Ana, their newest support whose heals can all miss, but the general idea is still true: interacting properly with allies can already be very difficult outside of organized play, and the less responsive a champion's utility, the more frustrating it can be to use. However, this is only one side of counterplay, and completely ignores effects that can be outplayed after they've been applied: Tristana's E is one such example, where her bomb always gets applied, but only deals important damage if Trist then follows up with basic attacks. Technically some supports have that in the form of boosts conditional upon their shields holding, but usually the shields are too strong and prolonged to be focused down in time, and the boosts themselves end up not being the main feature anyway: if the power on that were reversed, and the bonuses themselves were huge but the shields themselves quite small, then that kind of ability would have a lot more counterplay, as focusing the shield down to interrupt the bonus in time would be much more realistic. On a slightly different note, double-edged abilities like Kindred's or Bard's ults, which can be exploited by the enemy, also set a fantastic precedent for support abilities that could be both powerful and versatile, while still being fair.
To some extent, I think some of this can be applied to marksmen: because a large part does and always will come from a source of damage that is unavoidable, it might help to add more forms of counterplay to them that allow their opponents to legitimately react to their moves, instead of simply trying to kill them before they get killed first. Making more steroids conditional, rather than on-demand, could go a long way towards establishing this, and it could also help shift a lot of marksman power towards mechanics that are more interactive, and therefore less reliable. Generally, reliability is the biggest issue with marksmen, and the fact that they have so much reliable power in their basic attacks means they would benefit from being extra unreliable with their abilities. Simply tacking abilities from other classes onto marksmen doesn't really work all that well, because most other skillshots, dashes, etc. are predicated on the fact that the champions wielding them have only limited reliability, and so often need to be able to have some consistent mode of damage or movement. If a marksman misuses their abilities or gets outplayed (which presumes that their abilities can be both misused and outplayed), they should not be expected to succeed any more than any other non-marksman who does the same thing, in spite of their autoattack-based power. This would likely involve shifting some of the raw damage marksmen currently possess off of their basic attacks, and instead placing it onto their abilities, which should still complement their autoattacking power, and as a result that could hopefully also create better diversity between marksmen, who currently all tend to do the same thing by dint of having most of their power concentrated on a mechanic that changes little from champion to champion.
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and 
and ulti but besides that she sits behind THE BACKLINE so half the time so she's out of sight range. No way in hell is it reasonable for a champ to let you autowin a teamfight if you don't always take her out first. That alone is way too much raw power for a support champion.
