I'm curious whether you knew that for sure when you wrote this post, or if you just assumed that because they were younger again. I'm early on in my second campaign, so I don't yet know firsthand if relationships are reset. The only other alternative is to vastly slow the rate of aging, but I like that we see them throughout their lifespans. Continuing their saga would mean that by the end of the fifth campaign they'd be like 200. I mean, what's the alternative? All of my original characters were 50 odd years old at the end of the first campaign. And of course given that they're not fully experienced, they don't have all of the abilities they gained throughout that life. When you recruit a legacy character, their age isn't reset so much as you're seeing further adventures of theirs earlier in their lives. It doesn't really make sense to me, and although I haven't yet dug into the legacy system (I only just started my second campaign), it doesn't really give me the feeling that I'm creating an ongoing saga for these heroes - it's more as if I have a stable of clones to recruit from. Their relationships are forgotten and their age is reset. When a legacy character is recruited for a new campaign they are a new person with the same name and appearance and some of the same skills.
It's not really the case that you can continue a character's story over the course of several campaigns. Having just finished the first campaign, I'm actually a little bummed to find out how the legacy characters work.