For me, the issue about 1840 is mainly that by that point in the series, I had enough of all the time travelling. Every new storyline was taking place in a new time period. I was tired of being introduced to all new characters who would be gone in a few months. I wanted to see Liz, Roger, Carolyn, & the rest of the present time characters. After 1897, the present time became a temporary stop over.
But the present-day family had been reduced to a collective damsel-in-distress. Was there ever much of interest going on with them for most of DS? They never even had a clue what was going on, so they couldn't really take part or react.
1840 seemed to me to be a correction of a mistake the makers felt they'd made, in ever leaving 1897 at all. They never actually left the past, once they'd gone back to 1840, though there were vague plans to eventually. I can understand, if they felt this way. The past was when DS woke up, and the magic came back, no matter how many stumbles were made along the way.