First, pitchers before 2000 should have greater longevity, because the game was different, velocity was much lower on average so batters didn't have to have the bat speed required in today's game, the Red Queen effect, and stress on pitchers' arms was less.
Second, Grienke averaged 91.3 last year and has gradually lost 1 MPH the last four years.
Smoltz is a complete outlier, and he was still throwing at 92.7 in 2007 when he was 40 years old, didn't fall below 92 until he was 42.
Justin Verlander still averages 93.7 at age 33
Hamels at age 32 last season averaged 92.6, and is a lefty, has actually added velocity the last few years
Jon Lester at age 32 last season averaged 92.0 and has shown no decline the last four years
Jake Peavy's last elite season was at age 31, he's been a #3/#4 since, his velocity collapsed after 2012
Mussina was a junk balling lefty at the end of his career, last elite season was age 34, after that he was a #2/#3 until age 39.
Dinosaur pitchers on your list:
Bob Welch - last good season at age 32, after that he was basically a #5 starter
Jim Bunning - remained a top pitcher until age 38, but he was pitching in the 1960s, the "dead ball" era
Bret Saberhagen - last elite season at age 30 after two injury plagued years, bounced back after injuries at age
34-35. Only started 30 games one season after age 25.
Basically your list proves my point, it's highly unlikely that an aging pitcher losing velocity will pitch at an elite level the rest of his career, Grienke probably a #3/#4 the rest of his contract if he doesn't break down physically, he may have one bounceback season where he pitches at a #2 level.