Look at this from the other angle, how many examples are there of smaller companies growing large by supplying saas to larger companies.

Look at the customer list of basecamp, any of their large enterprise customers could have built their own pm service, they didn't. Look at salesforce, how many companies could have built their own contact management system, they didn't. Wordpress, yammer, the list goes on and on. As long as you've got the right business model, you'll be fine.