I don't think open sourcing the store would significantly cut their sales. Running a world-wide package distribution network in itself is an incredible cost which many companies are willing to pay for. CentOS did not cut into the sales of RHEL either, for example. It's useful to have someone to yell at when things break.

Also note that there are already alternatives. I personally know a few companies who built their own stores internally and it's actually very easy to do so if you install the apps using side-loading.