Deciding when to abandon an ecommerce platform

We've been using the osCommerce platform (ms2.2) since 2005 and have done quite well in adapting it to do what we want. We're now processing a decent volume of orders with it and quite frankly the system is becoming a bit of a mess in terms of code and extensibility. We've done a tremendous amount of custom coding/theming and now we're starting to run across some difficult architectural issues such as the ability to properly build a kit/component item so our inventory isn't a total disaster.

I've thought several times about migrating to plaltforms like Magento but have been turned off by the complexity of the system and obvious performance issues out of the box. Well, that and we have around 10,000 items which are customizable so we're likely not talking about an easy migration.

Looking at the pro's and con's basically gets me this: a clean platform we can properly extend. I could probably save over $100k the first year in alone in providing more accurate shipping costs to customers. Inventory will be more accurate with kitting/components. I'm having some 3rd party integration issues with PayPal/Google checkout that could be fixed out-of-the-box with Magento. Magento has some other features which we don't currently have that might help a bit as well. That and the future of ecommerce most certainly won't revolve around platforms like osCommerce. We could use the Magento platform and leverage proper plugins just as we did as a startup with osCommerce and the community contributions.

So how do you make the decision? When is it time to ditch the old platform for the new?