Let's look at the recent history.
Rams 4 winning seasons and put together a great roster. The Lions are a shit show & might have the worst roster in the entire NFL. Don't forget, those who are ultimately responsible for the product on the field are still in charge (owners and upper management) since they were the ones who hired the previous GM.
The Rams were willing to take on an aging QB that has never won a playoff game and appears to be in the beginning stages of being injury prone, to get rid of a younger QB that has playoff success. They were so willing to get rid of him that they threw in 2 first round picks and a 3rd.
You must think that the entire Rams organization is full of idiots (except for the one guy that left for the Lions) and the Lions front office is full of geniuses.
I am quite certain the Rams made a massive mistake. Probably McVay became overly full of himself, but whatever the reason, in 3 years the Rams are going to be in big trouble and bottom feeders and McVay will find his seat extremely warm.
When was the last time Stafford stayed healthy an entire season? Why is that important??? Because the Rams OL is not very good. They have basically bet that they will win the SB in the next 2 years...that is not a very smart investment, IMO...and they will not find the success with Stafford that they think. The loose friendship between McVay and Stafford that resulted in the Rams getting fleeced in the trade will come back to haunt them.
I would have said the same if they had done a similar trade for even Brady, though Brady would likely find ways to win that Stafford does not have within him...but even then the Rams are not winning a SB in the next 2 years.
Now, are the Lions geniuses? That remains to be seen, but getting what they did from the Rams has been referred to by some experts as the "Rams Herschel Walker". While Detroit did not pull off THAT level of fleecing, it is a good way to start building a winning team. If they achieve 1/3 of the success the Cowboys did, well that would hypothetically equate to 1 Super Bowl win. Sorry if I am hopeful that they will make that happen, and whether Goff is the QB in that or not is pretty immaterial to me.
That said, he has lead a team to the Super Bowl, how many QBs in 2021 will be able to say that? Brady, Rogers, Mahommes, Wilson, Garoppolo, Ryan, Foles (backup), Newton (while he might play in 2021, will he be benched and become a backup?), Worthlessburger (unless he retires), Brees (unless he retires), and Goff.
In 3 years, it is likely Rogers, Brady, Ryan, Newton, Worthlessburger, and Brees will not be playing. Meanwhile we will likely see Rogers going to 1 more SB and Mahommes to 1 or 2, so the list of SB starting QBs will likely only add up to 2 new names at most...and that list will not include Stafford.
While it remains to be seen if Goff will return to his earlier form, there is at least potential provided the Lions team improves overall, which with 5 R1 picks in 3 years they should be a considerably better team. If those picks are used on a WR, DL, LB, CB, and S...that alone would be great improvement and that does not include all the other draft picks and FA signings that will completely change the Lions.
Goff by then will more likely be a seasoned vet who is leading the team to playoff wins and potential SB victories than being replaced. A down year for him next year is projected, but the Lions are still more likely to win more games than they did last year. After that, expect him to be leading them to the playoffs.