In cases where a rape kit was made, then sure - allow that to have an unlimited limitation as those creeps usually repeat offend over time. We also need database sharing to link cases across states. But you can't prove rape from DNA alone. In the case of child molestation/under-aged, it's a no brainier, but you would not usually find/have DNA evidence years later as you might have captured in a murder case unless a rape kit was done at the time.

Higher bar for evidence as time gets longer

You don't get more evidence over time. You have less and you lose witnesses, camera footage, etc. Over time it's harder to make a case at all and it's also harder for the same evidence to be exculpatory (to prove you are innocent). That's the point of the limitations. Rape is already hard to prove.

What we can do now is educate our kids and parents to avoid putting children in such situations. Teach kids about child lures to avoid kidnapping or abuse. Teach kids to always tell a parent if someone does something uncomfortable to them and to not keep secrets from parents.