Dragon Quest isn't archaic at all, like the OP mentioned each games have their differences that make them unique in the series. the OP mentioned scenario but I'm going a step further and mention gameplay changes that people somehow ignore, when they shouldn't.
DQ1: This one is obvious, It was revolutionary back then. The game started the whole rpgs on console thing.
DQ2: Introduced battles with more characters to control, a bigger world to explore, more spells. You could say, DQ2 builds upon DQ1 and made the whole thing bigger and better.
DQ3: the changes are more noticeable here. you create your own party, DQ3 also introduced the job system to the series and to me the best class system in the series yet. Thanks to the class system in the game DQ3 was a very customizable game, you could have a Priest with spells only a Mage could get for instance. battles changed from 3 characters to 4.
DQ4: Introduced the Caravan to the series. You could switch party members whenever the Caravan was around. it also introduced an AI system for battles, in DQ4 it was forced, sadly as the AI wasn't the smartest. the doesn't take away the fact that they were already experimenting with AI.
DQ5: Introduced the ability to recruit monsters and have them help you in fights. it was fun recruiting monsters. Pierre and Golemuth were awesome.
DQ6: Totally revamped the Job system from DQ3, making it completely different in the process. no longer would you get sent to Level 1 whenever you switch jobs, instead the game encourage the player to switch jobs and experiment with unique set ups.
DQ7: builds upon DQ6 Job system, making it a lot more complex. it also introduced the ability to change into a monster to learn its abilities. DQ7 also introduced the Hybrid Skills to the series.
DQ8: Introduced the Skill System, after you gained levels you would get skill points which you could use to distribute to different categories each character had. also introduced the Tension mechanic in battles.
my point is, that DQ not only presents different scenarios it also sports big gameplay changes with each entry in the series, while retaining the basic gameplay mechanics that make the series so good. that's what a series should be, imo. When I play a DQ game I know exactly what I'm getting into, and that's a feeling most series do not have. that allowed me to fully enjoy every entry in the series.