nadenitza: gameplay is the emphasis here, gameplay - the old thief titles are just a proof that you don't need great graphics to have a fantastic game. Sadly more and more nowadays graphics outweigh gameplay...
Arkose: It's not that the old Thief formula wouldn't sell, it's that Square Enix are obsessed with spending enormous amounts of money on a game and then need it to make all of that back.
Tomb Raider sold 3.4 million copies in the first month which Square Enix considered a failure--not because 3.4 million copies is a bad amount but because of this generation's ridiculously bloated budgets.
This is why Thief and so many other sequels/reboots are veering heavily towards being "cinematic", action-oriented and dumbed-down. Initially it was just because companies were greedy for more money, but the changes they made in pursuit of profit to make the games more widely appealing also led directly to increased development costs, so now things are at the point where they actually need (not just want) their games to sell many millions of copies before a game actually starts making what they would consider an acceptable profit.
That sounds logical and when you think about it it's a trap devs fall deeper and deeper, probably the reason many filed for bankruptcy in the recent times...