Scarnecchia began his coaching career in 1970 with his alma mater California Western University as their offensive line coach. He moved to Iowa State University in 1973, staying there for two seasons as the assistant offensive line and assistant defensive backfield coach. Then began another two-year stint at a different college, this time Southern Methodist University as a graduate assistant before being promoted to offensive line coach.
From 1977 through 1978, Scarnecchia served as the offensive line coach for the University of the Pacific, before spending a year at Northern Arizona University. At the age of 32, he returned to Southern Methodist once again as their offensive line coach, spending two seasons there before following head coach Ron Meyer to the Patriots.
34-year-old Scarnecchia joined the Patriots in 1982 as a special teams and tight end coach. Meyers would be fired from the Patriots during the 1984 season, but Dante remained on the staff. Scarnecchia chose to move to Meyers' new team, the Colts, in 1989 to serve as their offensive line coach. But when Meyers left the Colts, Scarnecchia returned to the Patriots. He would remain in New England for 22 years, under six different head coaches.
Scarnecchia was the special teams and offensive line coach until 1994, even being temporarily head coach for the final eight games due to an ill MacPherson. Parcells move him to the role of special assistant, then became a defensive assistant one year later. When Pete Carroll was hired as the head coach, he moved Dante to head special teams. In Carroll's final season with the Patriots, Scarnecchia was moved to offensive line coach. When Belichick joined the Pats, he kept him at the line while additionally using him as an assistant head coach.
Scarnecchia chose to retire after the 2013 season and was replaced by Massachusetts native Dave DeGuglielmo. Even after his retirement, Scarnecchia still worked with the team, holding workouts for young lineman. After the Patriots parted ways with DeGuglielmo in 2015, due to the offensive lines collapse in the playoffs, the Patriots brought Scarnecchia back in 2016.
Scarnecchia has shown a special skill in player development. No matter where a lineman is drafted, he can turn a raw prospect into a star. One of the first stars he developed was Damien Woody, a center who occasionally played guard. He would be a Pro Bowler with the Patriots, but never reached that same level once he left Dante.
Then there was Matt Light, who was drafted in 2001 and became one of the best Patriots of all time. He helped protect Brady's blindspot for eleven years, bringing in three championships, three Pro Bowls, and even an All-Pro selection. Then just two years later, Scarnecchia developed late-round center Dan Koppen into an eight-year starter and one time Pro Bowler.
2005 saw the introduction of Logan Mankins, then Vollmer in 2009. I assume you get the gist. This development of star lineman has only increased, the latest star being Joe Thuney as well, as Isaiah Wynn appearing on the horizon.
Under Scarnecchia, the Patriots have literally not had an underperforming o-line unit besides 2008, but even that could've been due to the fact they were operating with Matt Cassel and not Tom Brady. Someone of his caliber is difficult to replace, but not impossible.
Carmen Bricillo joined the Patriots last offseason as a coaching assistant. He worked closely alongside Scarnecchia and could be seen as the favorite for the Scarnecchia's replacement. He doesn't have professional experience, but did spend nine years coaching the offensive line at Youngstown State beforehand.
Then there is Cole Popovich, who has spent five years with the Patriots. He was a coaching assistant who worked with the offensive line quite a lot. He became assistant running back coach last year, helping out the 65-year-old Fears. While you would assume he is being groomed for when Fears retires, it is also possible that the team moves him over to the offensive line.
Those remain the two most likely options, but that does not mean they will not decide to hire outside the organization. Many NFL line coaches would love to coach the Patriots talented unit, as well as work under a coach who has a knack for drafting offensive lineman. But either way, whoever is the next coach of the Patriots offensive line has some huge shoes to fill.