I think the traditionalism is probably one of the biggest reasons people like it. When i hear people talk about it they say things like "it reminds me of classic jrpgs" or "it reminds me of old Final Fantasy". And i agree it has many elements that i feel Final Fantasy has been missing for some time.
With that said i do agree that some things need to be modernized. the save system, and the music especially. I don't want combat system to change too much to the point it becomes an ARPG. maybe a real time combat system like FF12 at the most. I do think they could do away with the battle transitions at least.
Some changes would be welcome but at the same time I don't want such a drastic change where it barely feels like dragon quest anymore.