DAO -- Flemeth
DA2 -- Flemeth with Mythal hint
DAI -- Transition from Flemeth to Mythal
DA4? -- Mythal (IMO, she's still out there in some way, she's freakin all about shards of herself and nothing was conclusive about her being done before the extra credits scene; an extra credits scene isn't going to just finish off a major character in every way, sorry - she'll be back) and moving into her endgame?
Honestly that's not 'dragging along'. The Flemeth story itself may be largely finished, if not totally finished. We're onto Mythal now, if anything. So I don't think we have to care about an old lady in her hut in Ferelden anymore.
I'm okay with more Mythal. We've only touched on the truths of the Elves. Bioware can certainly give us more, along with more truths of Mythal (was she Andraste in some way? was the something before being an Elven Goddess? etc).
Threads to go from game to game are a GOOD THING. This Morrigan+Flemeth matter is a GOOD ONE. I enjoy it.
If anything was a more useless time of Flemeth, it was DA2. While I still liked some of the stuff of her there, it was much more of a cameo vs being important to plot.