I can't say anything for certain about OS's and partitions.
I use Windows, and often you can run very old applications on the latest Windows version. But yes at some point discontinuet applications perhaps will not work anymore.
I don't know much about MAC. But i think Apple often are doing things with the OS, so old applications no longer work on that OS. If it's a discontinuet product, that's not being updated anymore, then the product can't be used on that OS.
If you can run all your applications on the newest OS, and that OS will not be changed in a way where your discontinuet applications can't be used like you wan't to, then i see no reason to use multiple OS's.
In generel i'm not a fan of partitions. I would rather use multiple disks - or computers. But yes there can be circumstances where a pertition might be the best solution.
I have no reason to use multiple OS's if things work on the one i have and use. For security reasons updating OS can be important, especially as software often need internet connection to authorize. But yes there might be circumstances where multiple OS's can be useful.
If you use a DAW, then at some point you can get the same issue, if you update your DAW. It's the same for other applications.
What you wan't to do or need to do, i can't tell. The main thing is, it (if possible) work for you, whatever you choose to do.
Perhaps some who are exsperienced with Apple update policies during time, and about using multiple OS's and partions on Apple computers, can give you more informations, if you need it. I suggest you ask in Apple forums about that.