Maybe that's the way you see. I can't see it that way.
I personally admire what Solas and the Dalish are trying to do, even though I think they're mistaken in how they're going about it.
My answer to their struggles isn't "the past sucks and you shouldn't waste your time trying to remember or fix it. You should just forget about it and move forward without it." I feel that's throwing out the good with the bad. My answer instead would be would be, "Tread with caution. Stay flexible. Keep an open mind. Be willing to ask for help when you need it, and/or accept help when offered. Be willing to alter your course when you come across new obstacles and/or new information instead of trying to charge blindly through." But then you can say that about any endeavor.
(Besides, every culture in Thedas has negative aspects of itself worth forgetting, not just ancient Arlthan. The Chantry, for example, has encouraged magic-phobia against mages, racism against non-humans, sexism against men, and fundamental religious intolerance by branding all religions besides their own "false, heathen, and savage" for centuries. Should we disband the whole Chantry and dismiss it as "deserving of being forgotten"? Or modify/reform it? Maintain the good while fixing the bad? If all Thedas empires/nations/societies/cultures have flaws, I think the elves deserve to have their own rather than the ones that others push on them. As Merrill tells Anders: We have our own stories. I don't need to borrow yours.)
Besides, the Dalish can show a bit of flexibility on recovering the past. When Solas reveals what the marks meant in Arlathan, Lavellan can basically respond, "Whatever they were used for before, the Dalish have reclaimed them. This is what they mean to us now." And Solas will admit that despite what he's said about the Dalish, he admires that indomitable spirit.
And that's why my Lavellan chose to keep her vallaslin. She got it to honor her heritage, and she still thinks it's a heritage worth honoring, even if it's not worth replicating down to the last detail. And how would she know there were details not worth replicating if she didn't learn about it? She believes in learning about the past and educating the Dalish as much as possible and letting them decide for themselves which parts of the past are worth keeping and which parts are worth forgetting; which they can't do if it's all forgotten.
Oh no, I entirely agree with you that the Dalish are admirable for how they're trying to keep their culture alive and intact, despite how incomplete some of it is. It's a noble goal and it's worth preserving, as despite all the flack they get from people like Sera and some of the criticisms that Solas and Abelas give them, they are really the only bastion of that culture in the modern world that's out there.
As Lavellan can say to Solas in that first meeting when he comments on this, the Dalish aren't perfect, but they are trying.
Similarly, when you can call Abelas out when he comments that by taking the knowledge of the Well, it will be "lost" forever, by pointing out that by hording that knowledge and culture in a crumbling ruin, rather than sharing it with the elves that still exist in the world, it already is lost.
What I meant was, the Dalish in some ways are no different from Abelas and the Sentinels, in that they also tightly cling on to the past that is lost and are content to horde their knowledge from their kin, looking down on the "flat-ears" for being little more than Shemlen. They could be doing so much more to help, teach or reach out to the other elves, but they refuse to do so, out of pride and stubborness.
To be honest, that moment when Abelas rebuffs Lavellan is actually quite a nice little parallel to that Dalish's own attitude sometimes, as we're now seeing how much it utterly sucks to be on the recieving end of "I'm the true Elf and you're just a pretender".
I like to think that Lavellan learnt quite a great deal about the value of tolerance and acceptance of others over the course of the game, evolving from the relatively sheltered individual they were implied to be at the start (judging by some of their comments about how the clan avoided outsiders), to someone who's become rather versed in various peoples and cultures.