Really? I trust a private repo I am paying for to be there in a few years. Perhaps I am still of the "buying a cd" generation but someone somewhere has to pay for a saas service. It's just more real this way.
I guess if you depend on non-git features, like issues, there's a chance that there's some lock-in. However, most of the ones I use have a migration strategy [0].
[0] http://webapps.stackexchange.com/questions/49729/how-can-i-i...