IT guy here. I'm with you. I hate old IE. I wish it would die a horrible death. Having said that, I think new IE is quite nice, to the point that the only thing preventing me from switching is a few Firefox behaviors that are technically deprecated.
However, I must pipe up regarding old IE usage on corporate networks. In my experience, the thing preventing upgrading IE is legacy enterprise software, as you accurately pointed out. Sadly, these programs often were only purchased because they were the cheapest of the bunch. When you get products so cheap, you can imagine that someone isn't getting paid well, and you can surely bet it's not coming from the pockets of management. Thus, the development staff consists of people who are willing to work for $40K a year, and you can rest assured that they are not experienced developers. Sadly, they often aren't even the fresh-faced college kids with something to prove. To illustrate how bad this software is, I have seen an application that only works in IE9.. like IE9 specifically, not *up to IE9. It makes me cringe.