You cannot achieve getting all files opened in an MDI situation. They will be SDI whether you like it or not and you cannot overcome that.
THAT SAID, you almost certainly would like to do what you can to get the functionality you are used to (and that any sane person wants).
If just opened as files from, say, Explorer each will open in (of course) an SDI form in its own instance of the Excel program. Its OWN, unshared with the other similarly opened files. Those separate instances are, to Windows and Excel, as different as an instance of Notepad and Autodesk. They share nothing, and know nothing about each other outside anything Windows connected or sharing that being part of the MS Office suite of programs offers. So, let's say you want to copy and paste from one to the other. Copy a cell (a formula cell to show the difference) and paste it into an Excel file in a separate instance, it pastes the value, the same as it would if pasted into a browser or open-form document program like Notepad. Formula's gone, not pasted. Obviously one can use F2 to transfer the formula, but one at a time, eh? This kind of difference makes a noticeable difference with VBA too.
However, if you alter how you open the files, you can get a lot of the connection back. Say you have an instance open and a file open in it. Use File|Open to find the next file and open it that way. They are still SDI, nothing's changed about that, and both have a quarter of the screen filled with the menu (oh, sorry... um... Ribbon... if it walks like a menu and talks like a menu, it IS a menu), etc. However, now you can, for example, copy and paste in the full sense between the files. Formulas that work with references to other files only if the other file is open work. They don't in the separate instance example. It is NOT what you are used to, but it is a lot of it. And something you mention, the windows all over the place, doesn't happen so much. Another way to open files in the same instance is to find them in Explorer, say, then drag and drop them on the Excel program window. However, although MS fixed that, it's not completely conceptually fixed: try it with an attachment in Outlook and it won't do it. Want to open something from Outlook? Separate instance, i.e., separate running program, no matter what.
Another thing you can do to get some of the old functionality is to open Excel from, say, a taskbar icon, and open a second window with Shift-Click on the same icon. Those are linked, still the single program instance. Just clicking on the icon would have given you that second program instance and no linkage. By the way, some folks say it has to be Ctrl-Click. Well, not for me but I believe they are telling their experience and that which works for you will vary by the Windows version you have. Easy to tell which works for you, I think: for me Ctrl-Click does nothing particular, so presumably only one will work for you as well.
Personally, I find the Outlook thing extraordinarily aggravating and terrible. But outside that, they have restored almost all the things I need to make it through the day. I don't window things, unless you want to claim a full-screen "maximized" situation is a window (and I don't... argument for some other day), so I don't have aggravations with the Ribbon taking huge chunks of screen when I am using 3-4 files. Sometimes that hits me, but not often nowadays. If I used macros via the Personal workbook all the time, rather than just liking them available and using them sometimes, the separate program instances could be obnoxious, but I think only if I were regularly modifying them or adding to them so...
So explore it, use the above, and a lot of your ease of working may be available to you again. But nothing can make it the same again. MS has dig their heels in about the wonders of SDI (just like they did around 1990-1995 when they said how horrible it was and what a wonder MDI was as they replaced it with MDI). Assuming that has a natural course, it will be about 2035+ before they bray about how wonderful the "new" MDI they just ganked you with is compared to that primitive old SDI they used to have. I'll be dead and never know, but it fits. So get used to it. Oh, forget User's Voice. It's a pit, and they've already told people to pack sand about SDI. According to them: it's here, it's not queer, get used to it. Period. (So you won't be wasting your time getting back on track only to see next month's update return you to MDI.)