You don't even have to be 30. Eckler is 27. Josh Jacobs just 25. Saquon Barkley is 26. These guys were all franchise tagged, with none of the teams interested in budging. It's quite possible all three will be with different teams in 2024. My guess is 2023 will be McCaffery's last year in SF. It's a money and time issue.
Teams have a real concern of tread on the tires being thin. Several cases in recent years of teams doing things like paying Zeke that 6-year, $90m contract, only to find out that by year 3 things didn't look so great, and by year 4 he wasn't even close to being the best RB on the team. You could make a sizeable list of RBs that got nice contracts, and the numbers prove it takes like 2 years before the guy is beat up, washed up, call it what you will. Devonta Freeman, Todd Gurly, David Johnson, Clinton Portis, Lev'eon Bell. Remember how the Seahawks franchise tagged Alexander, then paid him a whopping 8 year $62m deal, at 29, thinking because he'd never been injured... RBs are frequently overpaid. Even today it still happens. The Vikings gave Dalvin Cook $12m. He still gained 1,100 yards this year, but his average is steadily dropping, on a team with a pass-heavy attack. Joe Mixon is scheduled to make over $12m each of the next 2 years. He ran for 1,200 yards in 2021, but just 800 last season. His Y/A dropping too.
The other issue is that college is loaded with running backs, loaded. Few that come into the NFL are that elite, and can last a while. Maybe one guy every several years. The rest are almost plug-n-play, where the OL does most of the work. Seattle has seen this several times in the last decade. Grab a good RB in the middle rounds, plug him into a system that run blocks well, and after he's worn out, put the next guy in.
I realize this is kind of ugly, but it's the nature of the business. The NFL is a passing league. QBs, OTs then WR on offense get $. On defense it's DE/Edge rushers, shutdown CBs, and some LB or DT who can pass rush. Pay money to pass on offense. Pay money to stop the pass on defense.