I question the assumptions that there is something like 'modern adventures' at all, or that 'old school' adventures are more difficult or illogical. They're all the same and all different. There are and were these kind of adventures and those kind of adventures. I don't even say "good" or "bad" ones because as Arteveld demonstrated it all depends on your preferences and point of view. Sure, some more recent adventures tried to improve on the design and some maybe made it worse, but IMO the changes are minimal and p&c adventures are a more or less stagnant genre, even if still enjoyable. For me LucasArts adventures at their peak, like Monkey Island 2 and Day of the Tentacle still beat all others, old and new, but that's just me.

You have to understand the evolution of genre.
Sierra games originated from text-only adventures, which had dead ends, sudden deaths and all that stuff. Basically Sierra simply changed adventures to graphical adventures and started to expand them.

LucasFilm (as it was first called) chose and created a different path in evolution. They eliminated the parser with clickable keywords, which made games much dumber as you saw all verbs at one glance, no need to think of anything that wasn't part of the UI. And programmers didn't need to anticipate user input, etc., which made game development that much easier. This also made the games less challenging, but more user-friendly.

Majority of gamers prefered easier gameplay, which is why in the end most adventure games got simplified to no dead ends, single icon cursor, visible hotspots, etc.

But in their own time mazes, deaths, pixel hunting, overlapping but not interchangable verbs (open door vs. use door) and all other annoyances were considered a good game design.