The de facto for any qb taken in draft is he starts on the bench. There should be no automatic path to starting. If fields is traded over summer, that draft pick is starting by default, unless you bring in Andy Dalton Jr.
If Fields starts the season and succeeds, great. You don’t have to trade him. You can play out the season with the pick on the bench.
If the rookie takes over the job, great, he should be able to eventually, but not necessarily preseason. Then you have Fields as backup in case of injury and/or trade him to a good team with a dead qb midseason.
If you can get a 1st in the summer, by all means go ahead and deal Fields. But if your settling for the likely middling market for Fields, you might as well keep the insurance policy in your back pocket.
there is no deadline to trade fields. You don’t have to do it. It’s not dumb at hold on to a talented qb