That's a bold statement!
There's no doubt that some speakers using plastic cones can sound soft in the mids. But all of them?
Obviously I haven't heard all of them, but my first experience was when I was 17 years old and fitted some Pioneer, paper, full-range speakers (whizzer cones) to the front doors of my first car (Datsun Cherry). After a while I decided to upgrade to the next level on the Pioneer ladder, and bought some coaxial 2-ways with plastic coned midbass speaker. I couldn't work out why the more expensive speakers sounded soft, and worse than the cheaper ones (although a bit more detailed). In the end I fitted them in the back doors.
Next experience was living with some budget (£180) Kef Coda 8 paper coned speakers for a few years. Whilst owning these speakers I'd been buying all the latest hifi magazines, and reading all the reviews. Dynaudio audience 50se (plastic coned speakers) had been winning awards left, right and centre in every magazine. I went to a couple of local hifi shops to ask their opinions and was told in two different shops that for the money (around £500 or £600) nothing came close, and nothing else was worth listening to... I bought some, but although they seemed more detailed, over the coming weeks I found myself listening to less music. One day I plugged the old Kefs back in and the music came back to life.
A few years later I bought some Tannoy System 10 DMT studio monitors (plastic cones). They were awesome at producing sound at low volumes, had vocals were to die for, and also went loud as hell without distortion. Play anything with electric guitars (heavy metal type music), and it was a bit boring.
I also listened to some newer Kefs a while after owning my Coda 8s for a while, but I can't remember the model (mid 90's coax floorstander that got great reviews). They had white plastic cones and were supposed to be the latest and greatest thing, but again sounded soft and dull.
One day I put all these experiences together and realised they all had one thing in common (plastic cones). I don't care if people believe me or not, but It's highly unlikely I will ever buy plastic coned speakers again.
Here's a quote from speaker designer Paul Carmody "
Peerless 830657
For the mid, I knew I wanted a paper cone, because they tend to be best at reproducing electric guitars. (Why is this so? Less odd-order harmonic distortion? Smoother Frequency Response? I don't honestly know why paper cones make more realistic-sounding guitars, but my best guess is that they tend to have cone breakup that is similar to that of a guitar speaker.) I also knew I wanted a "large" mid, at least 6" in diameter--but, it had to have a fairly smooth Frequency Response so that I could cross to a tweeter at 2000 Hz or higher without problems. For less than $50, pickins were slim. I took a gamble on a Peerless 830657 6.5" midwoofer from their SDS series; and it turned out better than I expected."