I don't think age has anything to do with it.
On topic. Though i think it is important to differentiate between "hard" as stat bloat and "hard" as you need tactic skill to win opposing enemy which is often smarter than you.
So for example hard as stat bloat means that for example enemy has 1000health and i do like 10 damage
where other one is for example fighting with elites in JA2 where both sides have more or less same equipment and stats but player needs to literary outsmart enemy, using skillfully landscape, sight, tactics and sometimes punching hole with explosives trough building is best option.
DOS is partially 1 and partially 2. 1 beacause its hit chance is heavily based on lvl where 2 because it us super important to think how to position your characters, take into account what if situations and so on.
I disagree; I think the age of the game has a lot to do with it. The games from the late 80s to the mid 90s were extremely difficult by modern standards, but that time period is now 20+ years ago. If someone reeled off a list of games released even in the last 10 years that were as difficult as D:OS then I'd be a lot more willing to concede the point.
As to the health adjustment in D:OS: its not the ideal solution by any means, but it does affect tactics significantly. DOS can be very unforgiving in the 1st round of combat (when you only have your start ap pool), when all enemies are still likely to be active, and when you're in a weak starting position (e.g. ambushes). With the much smaller health pool on hard, the chance of one of your characters being taken out in that opening round is quite high, and the AI is good in that it does seem to focus fire on targets reasonably well.
I'm also not sure if you can even run the same builds on hard as normal; the classic example being glass cannon which is a huge liability on hard - if you play a non lone wolf game, your total penalty for hard + glass cannon is 75%(!). I'm currently in the process of restarting (again lol) with a 4 character hard party (rather than my 2xlone wolf current run) and the hp penalty is much more noticeable without the 70% lone wolf bonus. Even the tutorial boss fight can knock you down to single-digits hp if you get unlucky with the RNG.
I'm not saying that Larian went with the best way to scale hard; I think ideally they would have looked at mob ai rather than hp bloat (I don't know how they do mob ai, but I wonder if games could be designed so that mob ai was parameterised, and hard difficulty would then be a changing of the sliders to make mobs more aggressive, more focused on taking down a single target quickly etc perhaps).
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