Going back 20 years, not a lot of pure backup QBs have gone on to prominence, but there were a few.
Acquired by Trade:
The Three Matt's
Matt Hasselbeck (3 Pro Bowl Selections)
Matt Schaub (2 Pro Bowls)
Matt Cassel (1 Pro Bowl)
Acquired in Free Agency:
Marc Bulger (2 Pro Bowls)
Trent Green (2 Pro Bowls)
Jake Delhomme (1 Pro Bowl)
Tyrod Taylor (1 Pro Bowl)
Yes, there were some notable missteps along the way (Charlie Whitehurst, Tarvaris Jackson, Kevin Kolb, Matt Flynn, Brock Osweiler to name a few) . . . but I don't think the hit rate or miss rate in signing a back up is all that worse than drafting a first round QB.
Garoppolo is stuck behind Brady. If the Packers several years ago opted to stick with Favre and traded Rosgers, would that have been considered a bad trade for the team acquiring Rodgers? I am not suggesting Garoppolo is on par with Rodgers, but sometimes players get behind a player and it's not their fault they can't get on the field.
I'd say it's much worse as the 1st round QBs that "worked" out as the 1st round QBs that did work out fared way better than the 3 Matts (the traded guys, this is not a FA situation, since that requires no draft picks).
Cassel had 1 good season and while he made the pro-bowl, it was because of an unsustainable TD ratio (for him, more than 50% above his good year in NE). He threw for 3116 yards, which isn't exactly a huge number and was under over 60% completion rate. I have no idea if he was one of those alternates, but Roethlisberger missed 4 games and honestly, based on the stats, I would argue for Flacco as the 4th AFC QB (Brady, Manning and Rivers were all better and made the Pro-Bowl). Normally 3 QBs make the pro-bowl for each side, so again Cassel likely made it due to someone dropping out. He was under .500 for every other year after NE. Not exactly what you'd expect for a 1st round QB that worked.
Schaub - Not much to say. Decent QB, had 3 good years, but honestly, led his team to 1 playoff appearance in 7 years and became known more for his pick-6 ability than his QB prowess.
Hasselbeck - Only one I'd consider a solid QB for several seasons leading the team to the playoffs 6 times.
Here's the list of solid QBs in the 1st round (I'm going to include 2004 (Schaub's year and 2015 as well, no need to leave out the bookends) that you were trying to say isn't much better than the 3 Matts, heck even Bulger, Green, Delhomme and Taylor:
Winston, Mariota, Luck, Newton, Stafford, Ryan, Flacco, Smith, Rodgers, Manning, River, Roethlisberger
Honorable Mention (i.e. as good as some of the guys you listed): Cutler (1 pro-bowl), Tannehill, Bradford, Vince Young (2 pro-bowls), RG3 (1 pro-bowl)
Possible: Bortles, Bridgewater (still feel bad about that injury)
Summary is that the 1st round QBs that worked out are among the top QBs in the league and have been/will be their franchise's QBs for years, i.e. not just 1 good year. I don't think there is a single backup QB you listed that is in the class of the 12 I listed. I would deem that as way worse success record and that if I was a team looking for a franchise QB (not just a starter by default) that I would take the risk of a bust to also have the chance at another QB that could be in the list above.