Development speed is way more important than scalability. Like Paul Graham says, the vast majority of startups fail because they make a product no one wants or because they never launch a product in the first place. Faster software development means you figure out if your product is a dud faster, and you're less likely to run out of morale before you've got something substantial.
I've been working on my current (unreleased) project with Django and jQuery on Linux for about a month. Here's all of the stuff I've made use of from the Django, jQuery, and Python communities:
http://code.google.com/p/django-command-extensions/
http://github.com/dcramer/django-ratings
http://github.com/pydanny/django-uni-form
http://github.com/robhudson/django-debug-toolbar
http://github.com/alex/django-ajax-validation
http://orkans-tmp.22web.net/star_rating/
http://jquery.malsup.com/form/
http://plugins.jquery.com/project/ScrollTo
http://jashkenas.github.com/coffee-script/
http://code.google.com/p/modwsgi/
There's no way I could ever use a niche framework after having drunk this reusable code kool-aid.
Even if Django weren't scalable, if I really was on the road to being the next Facebook or Youtube, there'd be plenty of time and money to rewrite in something that could scale.