Exactly. SRT & VS were marginally better than SG, when you take total % of the matches they bowled better. So, it is not correct to say that SG is the best part-time bowler ever. ODIs are mostly played on batsman-friendly tracks, on which SRT & VS are better bowlers than SG. So, they bowl a lot on batsman friendly tracks.
You might again say, why bring the total % of matches bowled. Simply because that gives a better perspective for a part time bowler. As your averages, Strike Rates etc showed, SG is even better than Imran, botham & Kapil, on seamer friendly tracks. Is it really true? No. That is skewed towards SG simply because he bowls 5+ overs only when he is doing better. In matches when he is not doing better, he will be taken off even before bowling the 5th over. Kapil, Imran & Botham don't have a choice there. They have to bowl their quota even if they are bowling relatively bad irrespective of the pitch, weather & match state.
It is important to analyze the statistics with the right perspective.
On batsman friendly tracks, obviously the average, SR & Econ rate will be worse for bowlers (even for regular bowlers, for part-timers they will be even bad). That explains the worse average (career or 5+ overs etc) of SRT & VS.
In certain favourable conditions, which are comparatively low, SG is a better bowler, which most of us agree. But that doesn't make him the best part-time bowler ever.
It is very rare where an ODI is played on a spin-friendly track favouring SRT or VS. ODIs are either played on batsman-friendly tracks or on seamer friendly wickets.
The fact that captains often choose SRT & VS to bowl the 5th bowler's quota for India, proves that SG is not even the best part-time bowler in Indian team. SRT & VS bowl the 5th bowler's quota not because they are great bowlers, but are less worse compared to SG on most ODI tracks.
AK home & away in ODIs:
Mat O R W BB1 BB2 Ave Econ SR 4w 5w
home 90 819 3646 126 6/12 4/25 28.93 4.45 39.0 2 1
away 80 684.3 2991 67 5/33 3/47 44.64 4.36 61.2 0 1
ZK home & away in ODIs:
Mat O R W BB1 BB2 Ave Econ SR 4w 5w
home 34 276.5 1407 45 5/42 4/47 31.26 5.08 36.9 1 1
away 51 421.3 2146 72 4/19 4/42 29.80 5.09 35.1 2 0
Srinath home & away in ODIs:
Mat O R W BB1 BB2 Ave Econ SR 4w 5w
home 80 685.2 3143 103 5/24 5/41 30.51 4.58 39.9 0 2
away 62 545.4 2260 88 5/23 4/23 25.68 4.14 37.2 3 1
Which should prove that the stats are not that bad for a seamer in India, and it is very bad for an Indian spinner away from Home. This is the reason why I said, It is very rare where an ODI is played on a spin-friendly track. ODIs are either played on batsman-friendly tracks or on seamer friendly wickets.