It's easy to measure what percentage of your users have JavaScript disabled. For a consumer website, it's probably measured in the hundredths of a percent.
If you can make a change that increases the conversion for 99.97% of your users, even at the cost of ZEROING the conversion for 0.03% of your users, then you will still come out ahead.
Also, it's trivial enough to add a <noscript> tag advising users to turn on Javascript (with a friendly "here's how" link), or you can do the work to degrade gracefully. Your conversion might drop for such users, but there's no reason it should go to zero.
Another site I maintain shows numbers like 3% IE6 and 1% non-JS. What a difference!
Of course, for the first site I mentioned, we made it sure the site graciously degrades for non-JS and/or IE6 users. We don't want to lose 15% of our customers.