Here are the risk-reward scenarios, IMHO:
1) They spend the higher of their TWO 2nd-round picks on a QB with 3+ seasons in the NFL and some successful starting experience - a guy who seems likely to be competent at the very least - AND they get to "test drive" him for half a season while paying him peanuts.
If he works out this season, they sign him to a 5-year deal that's probably cheaper than what Cousins would demand. Then, they trade their high 2018 FIRST ROUND pick for more picks to fill other positions of need - whatever they haven't filled using their still-prodigious amount of cap space in FA.
They have a huge jump on the 2018 season.
If JG doesn't work out, they still have options #2 & #3 below, but they've only "lost" a 2nd-round pick.
2) Keep the high-2nd and try to sign Cousins in 2018. If the 'Skins tag him again, SF not only needs to pay him (probably more than JG),also they have to give up probably more than a mere 2nd-round pick to pry him away from the 'Skins (possibly their 2018 high 1st-rounder). This costs more $$ AND probably more draft pick capital and leaves them with a lot more uncertainty about their 2018 roster until next March or April.
3) Keep the high-2nd, skip Cousins, and spend their high-1st on a inexperienced college prospect who could (eventually) turn out to be another Goff or Wentz, or another Bortles or Tannehill. And, it's likely to be 2019 at the earliest before they know.
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Under the circumstances, the trade seems like a reasonable speculative investment from a position of strength. Even if JG doesn't work out, I don't think SF will have "thrown away" much of anything
