You're suggesting that the performance of Voges should be taken for face value and ultimately that he should be considered the best since Bradman the moment his arbitrary 20th test match is completed. He's great, but the timing of his career and where he bats in the order has a big part in why he's doing something incredible.
If you take Clarke, Smith and Hussey as reference of players batting at five or six and scoring god mode recently for Australia; the only difference is Voges hasn't had a slump yet. Each of them had multi-seasons of averaging Bradmaneseque at that position behind strong Australian top orders (Clarke had Hayden, Ponting, Langer and Martyn, Smith had Warner, Clarke, Ponting and Huss, Voges has Warner, Khawaja, Smith)
It doesn't detract from how incredible he's been but helps put into perspective why this particular 90+ average shouldn't be considered to the other 90+ average in Australian cricket.