Can't disagree enough, iPhone has proven that absolutely false. People are attracted to quality and performance. The M8 is gorky looking and felt terrible in the hand last time I picked it up.
iPhone's success isn't purely on how the device looks. It's a combination of the device, the OS, the service, the quality, their awesome returns policy, marketing and branding.
And your description of the M8 looks and feel is contrary to a lot of other people and critics. That's fine if you feel that way, but I don't think that's how the general public would feel about it.
If sales were just based off looks, HTC would be among the top Android OEM's in sales.
Sorry but to say looks wont sway people but marketing will...you have to show the product. In short the leaked photos show very little difference between the M8 and the supposed M9. The product (as leaked) is making people feel like a rehash. If you are purchasing a car and they sold it as the "new" model (not a refresh) and it has the latest tech in it buts still looks like a car sold five years ago that, would that be high on anybodys list?
I hold out hope but the M8 design didn't interest me and how many current M8 users are going to go "wow, I need that M9. Its better than my phone and looks the same I better ditch mine and buy that!". I have also went from almost certain to a wait and see what everyone else says after a month or two.
The M8 wasn't a monsterous seller. The general public don't even know what the One M8 is, so the One M9 looking the same doesn't matter - barely anyone will realise it's a similar design to a previous phone - and even if they did, it wouldn't necessarily be a negative.
If what you said was true, then Apple's 'S' model phones (4S, 5S) should have been rejected as rehashes by the public. Turns out the S models sold more than the non-S models (4S sold more than 4, 5S sold more than 5). So clearly, having the exact same design isn't the determining factor of success or not.
Again, the design isn't the issue for the M7, M8 or M9. It never was the issue. HTC's goal at the moment is to market and advertise the M9 aggressively so the general public will consider the HTC One when they want to buy a new flagship phone. They have the design and quality down pat. It's now about getting the message out there.
HTC isn't banking on One M8 owners upgrading to the One M9. They're banking on bringing in more, newer customers as their major priority, then having those who purchased the M7 upgrading to the M9 when their 2 year contract runs out. The M8 to M9 upgrades would be a very small minority.