Hi. I've just upgraded from Excel 2010 to Excel 2016. It looks like "nothing" I was used to works anymore. Especially

a) opening multiple files in one window - the ribbon is therefore repeated for each seaparate file - this makes things very dense for horizontal alignment and consumes half of the display with repeated ribbons in vertical alignment.
b) Arranging and navigating the windows - sometimes seems to work, but most of the time it doesn't. - With things not in place, I feel completely lost. The Alt+Tab dialogue becomes real mess and Ctrl+Tab dosen't work anymore.

I spend all day in Excel and am used to keybord shortucts. I need to compare files side-by-side and switch them quickly and intuitively.

From what I was able to find, it looks like everybody is saying: this is a new featrure introuced since Excel 2013, there is nothing you can do about it.

I find this unbelievable and incredibly frustrating. So please, anyone:

a) Help me undertand WHY this change was introduced. Maybe if I get the reason, I'll find some peace.
b) Are there any best-practices you use for arranging / navigating windows?

Thank you.