Top level Safeties are great into atheir 30's. Polamalu made a pro bowl at 32, Reed made one at 34, Lynch made one at 36! Dawkins at 38! Rod Woodson made one at 37, Carnell Lake made the pro bowl at 32, Darren Woodson made the pr uh.... played until he was 34.
If you're a safety who's skillset is intelligence, play recognition, and jack of all trades, then you will be good as long as your QB is. I bet 38 year old Dawkins couldn't run a sub-5 40, but that's not his job. Be smart, react to what you're seeing, make the play.
I fully believe in 5 years we'll be hearing about Earl Thomas being the best safety in football, and how he's built a hall of fame career just off of his after-30 seasons. I would trade a 3 and a player for him instantly even with Philly's cap problems. Give him 5 yr 50 mil, you won't find a better place to spend that 10 mil a year. He'll win you the games you're losing.
If you told me you'd give me a quality player replacement and equal draft stock for Fletcher Cox, I'd say maybe. If you said that for Ertz, for Lane Johnson, for Brandon Graham, I'd think about it. You could do a lot with 1's and 2's, and that replacement player makes it an easy transition. If you offered that for Malcolm Jenkins I would say no. Because we cannot replace Malcolm Jenkins.
"Fletcher Cox? I am aghast!" You might say. There are plenty of pretty good, pro bowl, moderately dominant DT's in the NFL. There's no such thing as a fringe-pro bowl safety. You have reliable, maybe even underrated starters, and you have all-pros. There is no in between.. Additionally, every single star safety has played on amazing teams. Look at that list of old guys I made. How many didn't play in MULTIPLE championship games. How many didn't play in Super Bowls? That's not a coincidence, they are the reason. Malcolm Jenkins is the reason. Earl Thomas is the reason. And he'll be the reason for the Lions, or the Bengals, or the Dolphins... whatever team comes to their senses and ponies up. Dallas has regretted a lot of moves they've made that were mistakes. This will be the one you publicly refused to make that everyone will bring up every time you fall short.