To manage 10,000 sites you need a way to update them all at the same time, when vulnerabilities appear, or you'll be in trouble...
You can use a special script to handle multiple wordpress installations, or just use a multi-domain script, like
Nucleus
Nucleus may be what you are looking for...
Believe me, I've thought much about this. To centralize and manage my shopping carts & affiliate programs I chose a script that costs around $5,000 , and it's not even for sale right now, to take care of existing customers.
To take care of my hundreds of sites I think I'll choose Nucleus. Free and good quality multi-cms.
P.S. I've tested almost all CMS that are around, and even considered one being sold for $1000 up to yesterday, no longer available too.