I think it is absolutely essential that D&D Next make the first few levels weaker and easier to manage. There are more reasons for that even for veteran players. (I've been DMing and playing the game quite a lot over the past year).
1) I'm sure that WoTC has received data that shows how a good number of folk (even veterans) want a chance to play simpler more frail PCs, at least to start). I think from what I've read (sorry I can't sight source) is that most D&D campaigns fizzle before the get to really high levels anyway and most of that occurs because the game gets too complicated. In my experiences over 30 years, I've always felt an urge to end a campaign before level 14 or so. It may be that my preference is to run simpler more frail PCs.
2) The game becomes more dynamic if low level play feels different from mid-level play, from higher level play, from super-high level play. This gives people the opportunity to develop the zero to hero, which again some people like.
3) It is far easier to add than to take away. For people who don't want the frail simple PC, the party can begin at higher levels, or the DM can even give more choices to the 1st level players. In the WotC threads someone came up with a great idea to let a first level PC give up 2 attribute points (either +1 in 2 abilities, or +2 in one) to take a feat right off the bat at 1st level. For veteran players that might be a cool choice (personally, I like variety in my PCs rather than pure power...so for players like me, that would work). Having to play 0 level or negative levels, or have handicaps placed on more powerful 1st level PCs marginalizes the choice to play weaker, simpler PCs.
4) In my experience with D&D Next, the simpler PCs at lower level have actually forced my players (some veterans and some newbies) to think more like their PCs and roleplay more in line with their character concepts (background, class and race fluff and crunch). When players don't have to sort out too many powers or figure out how to use too many abilities, they relax and they do things that the think their PC should do, not just what it says they can do on the character sheet. Once the campaign begins with interaction and roleplaying, for me, it is much more thrilling.
These ideas, plus others that people have mentioned makes it nearly impossible for WoTC to do anything else but make the first few levels simpler and more frail. I believe they are on the right track.