And lets remember how things work on the #1 smartphone platform (i.e. Android). New devices are released and they're behind the curve in the software department on day one.
A friend of mine purchased a Galaxy Note a few months ago and it's running Gingerbread. You know, the version of Android that was released in late 2010 - around the same time as the first WP7 devices were released. And it still hasn't been upgraded to ICS. My money is on Jellybean coming out for my Galaxy Nexus prior to him getting ICS on his Note as a OTA update.
And through all these update issues, Android is winning the Smartphone marketshare war. So I don't see Nokia Lumia 900 phones getting WP7.8 being a significant deal-breaker when it comes to trying to attract new customers to the platform.