Opensource the client.
If you may say "oh but is is bad for business".
Not exactly, capes and cosmetics are stored on servers, and have nothing to do with the client.
If someone wanted to get free capes there is already a mod to do so. So people would still continue to buy from the store.
"Oh but the anticheat"
Don't have to opensource that part. (seperate program running in background)
Reasons for opensource:
Bugs fixed alot faster, more features faster, seems more trustworthy.
Also allows you to add parts from other opensource projects as long as it is the correct license.