I've actually been looking at Plesk for years ever since we set up Lily, but obviously the control panel is the foundation of an entire web hosting company so switching is a massive undertaking. There are very few control panels that run on Linux and Windows. Some run on just Windows, and most run on just Linux, but not both. Plesk will allow us to have both Linux servers and a Windows server and it will tie everything together. I would love to go with free and open source, but as far as I know only a few open source projects have experimental support for Windows hosting so far.
As soon as I contacted Plesk asking them if they could help us, they literally dropped everything they were doing to get us a contract to sign before I changed my mind and looked elsewhere. Plesk, unlike cPanel, realizes that a verified and experienced non-profit with the purpose of providing free hosting is a gold mine for public relations. It's actually part of the contract that I signed that we have to let them put our logo and story on their website. This will make them look really good, and they know it.
Plus as a free host we attract a lot of people who are new to hosting, and have zero experience with any control panel. Over the years we have gotten literally hundreds of thousands of people hooked on cPanel. When they move on to paid hosting they make sure that cPanel is available before they signup because that is what they know and love from using it here. Now we can get all those people started on Plesk instead.
100% without a doubt cPanel thought that we couldn't survive without them, and we would just give up and hand over the $1442 per month to keep their licenses. Absolute pure greed. If the reason was anything other than money they wouldn't have just said "you can buy full licenses".