Ah the age (old) argument, had to come out sooner or later
If he were improving that might be admissable, problem is he isn't. It might also be backed up IF it weren't for the fact he averages 25.68 and 21.57 with the ball in ODIs and T20s respectively, 'age' doesn't seem to be a problem there
England can't afford to carry a bowler in a quartet, and they don't have an all-rounder so the chances of him being a passenger in a five man attack are slim - unless he himself suddenly starts scoring a la Prior and England make that suicidal move
Stuart Broad (Test bowling - progressive)
Innings 01-22 : 35 wkts @ 38.69 (SR 72.91, ER 3.18)
Innings 23-44 : 42 wkts @ 32.64 (SR 61.07, ER 3.21)
Innings 45-66 : 30 wkts @ 38.47 (SR 76.33, ER 3.02)
Probably better off sticking with the defences of 'creating wickets at the other end' and injuries/bad luck. Considering that is 22 innings bowled per spell and two out of three he's taken much nearer 1.5 wickets per innings than 2.0, he's not done that much.
Invariably he's bound to have had some good series, but if you look at his record by series it paints the same picture
Stuart Broad (Test bowling - by series)
vs SRI (07/08) : 1 wkt @ 95.00 (SR 216.00, ER 2.64)
vs NZE (07/08) : 8 wkts @ 31.25 (SR 63.00, ER 2.98)
vs NZE (2008) : 7 wkts @ 43.57 (SR 78.86, ER 3.32)
vs SAF (2008) : 8 wkts @ 49.00 (SR 82.00, ER 3.59)
vs IND (08/09) : 2 wkts @ 67.00 (SR 120.00, ER 3.35)
vs WIN (08/09) : 12 wkts @ 30.58 (SR 65.50, ER 2.80)
vs WIN (2009) : 8 wkts @ 25.38 (SR 38.50, ER 3.95)
vs AUS (2009) : 18 wkts @ 30.22 (SR 51.39, ER 3.53)
vs SAF (09/10) : 13 wkts @ 33.46 (SR 71.54, ER 2.81)
vs BAN (09/10) : 6 wkts @ 46.00 (SR 79.00, ER 3.49)
vs PAK (2010) : 14 wkts @ 23.36 (SR 48.79, ER 2.87)
vs AUS (10/11) : 2 wkts @ 80.50 (SR 209.50, ER 2.31)
vs SRI (2011) : 8 wkts @ 48.75 (SR 89.25, ER 3.258)
Only in six series has he averaged under 34 with the ball - four out of those six series against Pakistan, New Zealand and West Indies who were no great shakes as batting sides go. Against the aussies in 2009 it was two 5wis that dragged his average down, in six of his eight bowls that series he was typically mediocre (or worse) In six of his 13 series he's conceded at less than 3.00 rpo, so I guess at least he's creating wickets at the other end
"undoubtedly has the best years ahead of him" - given what's gone before him I find it hard to believe those were his best years so I guess in a sense you're right!