See the thing is, I'm actually getting really sick of being satisfied with the 'alright' we've become used to. So many spiritual successors or true sequels where everyone concedes that the game won't hold a candle to their predecessors but will be fine in their own right. I'm tired of being content with that.
Thief 1 & 2 are nearly flawless when it comes to their core gameplay and level design. True, rich experiences. Taking away or neutering any number of the original gameplay facets to 'streamline' it will have detrimental effects when directly compared to the originals. Yes, they probably won't have a detrimental affect on the game itself. Like we all say - it'll still probably be 'fine' or 'good' - but it won't be what makes Thief as good as it is. And that annoys me. That 15 years after the original was released, we're hoping for minimal damage to the series, rather than wondering what incredible new features are available to us as a result of new technologies.
It's all rather backwards.
For example, this 'focus' feature is just a single 'streamline' to the game that, in the end, won't make the game terrible or outright unplayable, but it's a big blow against what we were used to in the originals. Just by showing where the player should go takes away so much immersion, fear, intelligence, atmosphere and fun from the game. No longer are you casing a complex and mysterious mansion where it's up to you to get your bearings, figure out the layout of the place, read notes and overhear guards to get an understanding as to where the 'prize' might be kept etc. It becomes, head down, go straight towards that quest marker.
And in the end, no amount of spectacular engine or art design can save the game with implementations like this. It simply won't be as an immersive experience as the original games, dated engine, warts and all.