Will there ever be a natural successor to Thief...

What's wrong with Styx? It's a poor man's thief, sure, but it's a good game.

I'd like to add to this saying a vast majority of what made old stealth great were hardware limitations. Things such as perfect blackness of CRT monitors you won't ever get on big ass plasmas with consoles or 4K LCD with PC community. You can't design a proper game while dealing with dark grey/light grey shadows and tapetum lucidum perks people got used to.

Wrong.

Thief is an immersive simulation. The problem is that immersive simulation is too complex to design for the hack studios of the current year.

The closest thing to stealth in those games is just following people for five minutes in order for some sort of irrelevant lore to come up.

Not until sound tech catches up and a developer with a large enough budget uses that tech. Sadly your average casual does have good enough headphones or speakers to make good use of proper sound and publishers are only interested in marketing to as wide a group as possible: why spend money on sound if only 10% of your players will benefit? It doesn't help that the rights to the sound tech used in Thief and similar games at the time are locked away by the biggest jews in the industry.

Thieves Guild is one of the Gold levels and one of the worst in the game. Push through it and the game improves massively.

Thieves Guild is legitimately a terrible fucking level and I don't blame him for dumping it there.

I liked the pickpocket segments and finding out about your target and things you can use to your advantage like rafters to drop down on him. The few moments I was able to move in and assasinate him without being detected were extremely satisfying. Unfortunately the game doesn't really give you the opportunity to do that very often.
I'm talking about the first game, of course. The second one threw all pretenses about stealth out the window. I never played the rest.

...

user there are plenty of submarines games. Try Silent Hunter III with GWX for maximum Das Bootness.

It's the GWX manual that mentionted bottoming out doesn't help. Also I think only II had AI subs and multiplayer for maximum sneak.