Why does Apple care about people connecting their monitor to a windows PC? Thunderbolt is compatible with displayport and you can buy a dirt cheap mini dp to hdmi adapter. You know what's garbage? Dell not being able to make a proper docking solution since.....ever.
You know who uses thunderbolt? Video professionals that'll make use of 4K and the insane throughput of thunderbolt. All other interfaces is dog slow.
People that rag on thunderbolt don't realize that its an all in one plug.
You are partially true, so also partially wrong. Thunderbolt port on the Mac side is indeed convertible to DP, HDMI or DVI. That's why you can connect Mac with TB to any display, including this Dell display assuming you have the right adapter and connectors.
But Thunderbolt Display is NOT the same as you might realize. Its input is for TB computers exclusively although it looks exactly the same as mDP port. So even if you have the right adapter it just won't work. What kind of $999 monitor which does that? That could be misleading for average joe.
Okay Apple could not care about connecting Windows PCs to its own display. But how about pre 2011 Macs? Those are their own products, no?
Even Lightning has an adapter for 30pin dock so old cable and accessorizes can still be used. Why break the chain all of so sudden?
Yes, Thunderbolt is an all in one solution for video and data. But if you're professional you already have so many devices and cables already. Does it really matter to just save a mere one or two cables while adding the cost significantly?
You can still perfectly use TB port on a Mac while connecting it to a HDMI monitor (i.e rMBP, new MacPro and Mac Mini). No harm done.