I really like the idea that the Tarnished Symbol equivalent upgrades-in-place a Third Age to a Second Age, etc. without having to remake it. But the grind of swapping out an item every 5-10 levels is the bigger issue.
But while I agree with a lot of posts here about what I would like for me, I think it's pretty fruitless to talk about what I would like while dismissing what Turbine needs to make money. Because if we're gonna do that, what I want from the legendary item system is for Turbine to send me a check for one meeeeeeeellion dollars every time I get a LI from a random drop.
Generally, I think if we, as a community, could rally behind ideas that give both us and Turbine what we need, we'd have a thousand times better a chance of getting our ideas taken seriously. But we really never do that. We're not cohesive enough to do it.
For instance, if the idea of the Third-to-Second rare-drop item were to be considered, imagine if, at the same time as they added it, they also added a Buy It Only In The Store scroll that upgraded in place a min-level-N item (of any Age) to a min-level-N+5 item (changing it to Third Age). Those two things put together mean everyone can get one good item, pour time/agony/TP into getting the legacies right, build up relics using the timesink grind Turbine wants us to do, but then keep all of that and then upgrade the item by levels and ages -- one via the store, the other via raids, and never have to deconstruct-and-rebuild.
Turbine loses the money they get from selling the TP some of us spend on relic removal scrolls and some of the LI upgrade scrolls (delving, legacies, etc.), but they could well come out ahead anyway if the "add five min levels" scroll costs enough. And they still can offer us some timesink (but not nearly so frustratingly full of "everything I just did was for nothing") in building up relics and levels, and some dependency on raids to have the best gear, and they can still promise "you can do everything without spending TP, but if you want to spend TP you can get it easier and quicker". And we can avoid the most tedious and the most lore-breaking parts of the LI system (that there's ten thousand LIs, the ground is practically covered in them, and we throw them away like so much toilet paper when we're done with it). And it even doesn't require a lot of recoding, or break anything we have now, since it just adds one more pair of options to it.
But we'd never rally behind that kind of idea because of that one part where Turbine charges us money for the add-five-min-levels scroll. So instead we end up stuck with the mess we have and have to make the best of it. And we propose lots of wonderful solutions from our side of the table that are all money-losers for Turbine and thus are never even going to get listened to.