This is something else. Adventures are just different now. I feel like many of these old modules you spend 90% of the time in combat encounters, where as in most of my sessions we have one or two combat encounters. It's part of the game still, but it's no longer the core part of the game.
So I think this may be a misconception. Though a lot depends on the adventure and the edition. Certainly it's true that some of the later 1E, most 2E - 4E adventures often have a combat focus, early 1E and prior adventures, which are the ones that generally get praised and talked about, aren't especially combat-centric. Oddly perhaps one of the critiques of contemporary adventures in the old-school scene is that they only have combat encounters.
I think this happens because in early adventures all encounters are described with stats and the tools you'd need to make them combat encounters, and you have a system that doesn't have many mechanics for social interactions. Yet, a few things work to make it so that many of these encounters won't be resolved through combat (for some people they will - but it tends to lead to dead PCs fairly quickly).
A) Reaction Rolls: Prior to approximately BECMI, the assumption is that the referee will roll for the reaction from almost any monster or NPC encountered. Usually a 2D6 with only a 2 being immediate attack. Other creatures tend to warn the party off, threaten them, or demand something rather then attacking immediately. This means that it's fairly rare that a combat doesn't start with the PCs decision to engage in it (well 2 in 6 surprise ... so it's complex).
B) Morale Rolls: Morale means combats tend to be shorter, and if one is fully engaging with the mechanics can be pushed towards the enemy fleeing or surrendering with things like impressive displays of magic or trickery.
C) Schemes and Skullduggery: There is an aspect of older play (or perhaps just OSR play - but I think it starts early) of evading, tricking and assassinating monsters - or otherwise throwing things into the party's favor so that they can face the minimal combat risk. This means leading monsters into other monsters, using faction rivalries in the dungeon to gain allies, setting traps, using illusions or other lures, finding good ambush spots or simply bribing monsters.
Of course none of this tends to be in the older adventures explicitly. It looks like a constant battle, because these are not highly supported by mechanics. Instead we have only the play reports and table culture of old games as well as some indications of how scoring in AD&D tournament modules worked. For example the version provided in the A series of tournament modules is based on how many regions of the adventure the party completes, how many PCs survive, and some bonus points for clever listed schemes. These are not individual rooms, but areas of the adventure, so the best tactic is to move through them without too much combat.
Now, if converting these adventures, it makes sense to say "So What?" I think overall you are right to shorten the map, reduce number of encounters and otherwise simplify for 5E - because as I mention, it's aimed at being a different sort of different game. I would characterize 5E's intended playstyle as cinematic heroic adventure, 1E isn't that - its characters are anything from game pieces to nefarious schemers and opportunists (they can also be heroes - sometimes all three at once), its adventures are picaresque and its focus is on outthinking and unpuzzling the obstacles of the game world -- a playstyle that at least partially evolved from the mechanics, rather then from the long history of the genre. Late 1E-3E and 3.5-4E have their own play styles, goals, and mechanics/rules as well, but the point is still that each game and its adventures can only really work well for it unless modified. Understanding how the rule sets and play cultures function for both the game being played and the adventure being adapted is useful here. Tricky I think but useful. Like I don't know how to write a great 5E adventure. I ran a game for a year, but my efforts floundered because my expectations were too deeply rooted in OD&D and I ended up fighting the system mostly.
