On the assumption that any such player will have 1000 runs and 100 wickets (a reasonable “threshold qualification” anyway) you are correctI selected batting average of 30 because I felt that is the point a bowling allrounder really crosses great utility mark. On the converse, an ATG batsman who averages 40 with the ball wouldn't add as much in his secondary discipline as he is an opportunity cost for superior bowlers.
If we choose to select similar quality in both disciplines over a period of time, (bowling avg of under 30 and batting avg of over 40), it will be interesting to see who qualifies. Probably none.
Records | Test matches | All-round records | 1000 runs and 100 wickets | ESPNcricinfo.com
Although obviously I assume there are a reasonable number of great allrounders players who may well have met the criteria until relatively late in their career.