Fields has shown NFL talent
He has shown he can run. That is all.
Honestly, running at that level is enough. If Fields never improves as a passer, he's still a good enough QB with that level of rushing prowess. If he improves even slightly as a passer, he's probably a top-10 QB. If he improves a good amount as a passer, he's an MVP candidate. Fields did a good job keeping the Bears in almost every game last year, they were 1-7 in 1 score games, including 50-50 games with Philly, Minnesota, and Miami.
Personally, I'm betting on the slight improvement. But Fields through 2 seasons, has shown as much or more than Josh Allen or Jalen Hurts did, and is on a similar path right down to getting a #1 WR in year 3. Not saying that is a reasonable expectation for Fields, but its in the range of possible outcomes.
Meanwhile with Lance (since its his thread and all) he's likely in a losing battle for a starting job, and I can't see any team offering more than a 3rd rounder for him in trade, which SF probably wouldn't do.
ETA: Which isn't necessarily saying Lance is bad, just that its been 2 years, and he's shown nothing, and while the circumstances were unfortunate, his time as SF's guy, may have already passed.