Gaines didn't start a single game his first 2 seasons and then blossomed in year 3. As 4th round pick in 2019 it took him 2 years to find his way to the starting DL.
The Rams, as they tend to do, drafted Gaines' eventual replacement a couple years before they needed him. They spent those two years preparing for what they anticipated would be Gaines' departure in FA after the 2022 season.
Bobby Brown III, also a 4th round pick (2021), is poised to follow in Gaines' footsteps. BB3 had 1 start in his first 2 years and at 6'4", 324 lbs actually has a more prototypical build for the NT position (compared to Gaines at 6'1", 313).
Obviously nothing is guaranteed, but don't go to sleep on BB3 quite yet.
AlbaNY_Ram