To be honest, I think you're massively overrating Ralf Rangnick while at the same time massively underrating the spending power he has had with his clubs in recent years. In their first season in the 2. Bundesliga Hoffenheim spent more than 18 million Euros on new players. That was way more than the rest of the league combined with loads of clubs having a positive net spend. Of those clubs spending more money on buying players than getting money for selling them only Koblenz had a higher net spend than 1 million euros (2,2 million to be exact). In total players for roughly 37 million Euros were bought that season by all clubs in the league. Half of that was spent by Hoffenheim.
The same can be said for Salzburg to a smaller extent as they seem to have gone for more of a feeder club approach buying younger cheaper players, partly developing them at their academy and then shipping them off either to Leipzig or the highest bidder. They're still able to spend much more money than the other clubs, especially on young talented players they can put in their academy first...
Leipzig have spent 18 million Euros on new players this season. There were only three other teams in the league who spent more than 2 million on new players (Freiburg with 4,9 million while selling players for more than 24 million, Paderborn 2,7 million and Kaiserslautern 2,6 million). The rest spent about 1,5 million or much less on new players. The season before, Leipzig bought players worth 23 million Euros. Only one other team spent more than 2 million on new players (Nürnberg with 3,9 million and again the sold players worth more than 18 million due to being relegated the season before). When they were in the 3. Liga they spent 2,88 million Euros on new players. The other 19 teams in the league together spent a total of about one million on new players that year.
Money is the main reason Hoffenheim, Salzburg and Leipzig are successful.
That said, I'm not denying that Ralf Rangnick is a good manager and I like him as a person. However, to say that he's the reason why those clubs have done so well in recent years when they've been able to massively outspend the opposition on a regular basis is a bit far fetched. Again, I'm not denying that he has done a good job spending that money, but it has to be much easier to do that, when you can afford to take one or two gambles on players when other teams are basically struggling to stay afloat...
And I am also not so sure about Roger Schmidt being uncovered from nowhere. I think Rangnick showed an eye for talent when he got him to Salzburg, but it's not as if Schmidt was sitting in a cellar somewhere waiting for a call. He had done well at Preußen Münster and did a decent job at Paderborn. That's probably the standard of German manager you can get in Austria. I'm not sure anyone could have predicted how things turned out for Schmidt in the end...