I agree with you in general. However, I don't think the problem is nearly as bad as the hair metal trend. While this trend is certainly making metal heads more acceptable of identical sounding bands, I don't think anyone can mock metal as a whole as a result. There are people that think of Warrant's Cherry Pie when they hear the word "metal", and the entire genre is ruined for them. Sure, I can have iTunes shuffle Epica, After Forever, and Within Temptation and not be able to tell which band is playing.
Funny ... I don't have any trouble telling these bands apart. As you've probably already noticed from the diverse bands listed in this thread, female metal singers come in more varieties than just this operatic/symphonic style of singing. Those might be the style you gravitate towards more, or the style that the bigger labels are pushing, but that doesn't mean that's all that's good or all that's out there.
Having singers and bands that sound similar is inevitable in any genre of music. That's the reason they are in a genre together in the first place - because they sound similar. Every band can't be in a category all it's own.
If you played three black metal bands for me I might say that they all sounded the same, but to someone who is a big fan of black metal they can probably pick up many of the differences in the music and the singers' voices.
If you played a casual power metal fan certain songs from Blind Guardian, Savage Circus, and Iron Savior they might think they are all the same band, but to the avid power metal fan each of these three has some unique qualities that make the band all it's own.
I'm just tired of people saying that female fronted metal is a fad, because what you are basically saying is that women singers in metal are a fad. They aren't. They've been here for 30+ years (Girlschool anyone?) and they aren't going away, even if they do "peak" at some point ... like every other phase/subgenre in the metal genre.
And to those that say this subgenre is prone to burnout or that there are no unique sounding female singers out there ... many female metal singers are actually diversifying away from the operatic/symphonic style, as many of them have to constantly put up with this repeated (and at this point, unfounded) criticism that all female metal singers sound the same. I'm finding that many of the newer bands with female singers actually take more of a power metal or bluesy approaching to their singing style rather than the cliche operatic style that so many assume as soon as the words "female" and "metal" are connected.
If anyone thinks that all metal bands with female vocals sound the same and that female singers in metal are "making metal heads more acceptable of identical sounding bands," then you obviously aren't looking very hard for anything that sounds unique 'cause there's plenty out there.
If you are into the operatic/symphonic stuff more, I recently came across this amazing symphonic power band from France,
Kells.
If you want something on the complete opposite end of the spectrum, but still great, I don't think I've seen
Holy Moses in this thread. They've been around for 25 years and their new album is really heavy, thrashy and the vocals are ten times better than anything from Arch Enemy imo! (Although I doubt you will ever see them at PP.)