Yeah, well there's no magic wand to convert old PHP scripts to be compatible with PHP7. However, you can manually convert them which is what I do with EHCP software. Usually, I just change software so that it is compatible with both versions to avoid any problems.
It does suck, but the blame goes to PHP (for their stupid ways of deprecating code and not converting these deprecated functions over to the new format), and that's all you can do.
You should run one server with several virtual machines. Then, it's no problem to run many servers on one server.