Additionally note that there are different types of certificates. The ones LE issues are domain-validated (DV) certs (as you indicated some CAs call this "PositiveSSL"). Certs like OV (organisation-validated) or EV (extended validation) are not issued by LE and also will not be issued in the future because you can just not automate this. So this is what other CAs can still do.
Note that only EV certs practically make a difference to the user, because they are the only ones that show the big green bar with the company name in the browser window.
There are no reasons more for buying DV certs. Some other (commercial) CAs also seem to have noticed it, because it is said that at least 2 mayor CAs also offer free DV certs in the near future.
Basically this means: Encryption for everyone! - And this is what LE aims at.
As I said EV certs can still be sold, so this is still possible for commercial CAs. Additionally as LE does not support wildcard certs this is also a type of certs which can be sold.
So it is not that all CAs do not have any possibility to get revenue anymore, but they may not be able to do it with simply domain validations (which only take a few seconds and require no human-interaction as you can see at LE) anymore.