Is there a way to make paste as text only the default action in Office 2016?
In a roundabout fashion yes, mostly...

In MS Office their version of "Paste as plain text" is called "Paste and Match Style and uses the macOS system defaults (OptionCommandShiftV) for paste as plain text for that function.

Now if you don't want to have to do that four finger salute just to paste (essentially) plain text, you can change the menu shortcuts in any application via the Keyboard Preference pane.


Click on System Preferences > Keyboard > Shortcuts > App Shortcuts
Click the + button and from the Application menu select (for example) Microsoft Word.
Enter the menu title that you want to change. noting that the menu that you want to assign or reassign it's keyboard shortcut much match exactly what the menu says down to the capitalization.
Click in the Keyboard Shortcut field and tap the keyboard shortcut you want to assign to that menu option, then click Add
Repeat for any other keyboard shortcuts you want to add/change in any other applications you want to change them for.


In this example I reassigned "Paste" to OptionCommandShift V. And "Paste and match Formatting" to CommandV.



I didn't do this for all applications as I don't want to chance messing up the behavior of other applications that you can copy/paste with.

NOTE that this is not really "paste plain text" but will give you at least some of that functionality with very little pain.

To really get paste plain text in an MS Office application you would likely need something that could capture a regular paste command and then select the plain text icon from the formatting menu that pops up at the insertion point after you paste. There may be a macro program out there that can do that but I am unaware of any specific ones with that functionality.