BVB’s most recent chapter of decline predated Tuchel though, as a matter of fact it started when Klopp was in charge. What I meant by Goetze was, Goetze was clearly one of their biggest stars at the time (along with Reus) who showed a lot of promises, more importantly he was the pivotal point of their midfield-front line therefore losing him was a big blow and the watershed moment for BVB. BVB has been losing players for years before that, for example Sahin and Kagawa etc., losing Goetze was probably the final straw the broke the camel’s back.
It is easy to blame their management for the decisions they made and what players they bought and should have bought etc. but let’s face it, BVB and many other Buli clubs are just a springboard/incubator to go to Bayern and overseas elite clubs. They don’t have enough money to match the salaries offered by the elite clubs and they simply can’t retain their players. And honestly, Bundesliga at its current stage does not look that attractive to top, established players (unless it is Bayern) so it is hard for the likes of BVB and Schalke etc. to outspend and match Bayern.
You have the problem to look first to the gap to Bayern - yes, that is impossible right now and actually was the recent seasons as Bayern did not have big blips. In all seasons since 2012 they had atleast as much points as the historical 1972 team had (who held their record for 40 years!). But Bayern worked for that position for decades - both on and off the field. It did not come overnight but the bright time in the last seasons really came from an exceptional generation with a real great mentality - prior Bayern teams thought they could win matches with 40% "investment" and that often did not work out...
Winning the league does not mean to have a strategy in that one or two matches against Bayern - but to have one against the other 16. And the Gegenpressing strategy of Klopp was great when it was new in the Bundesliga - and is against top teams that want to play with the ball - but it is a lottery against teams that are equally setup and you need another midfield for playing good ball possession football. And you are right - the problems already started in the Klopp era in 12/13 or 13/14 - even if their football still worked great internationally. The teams started to sit back and hoof the ball against them. They still had good results especially as they often had a match opener with a set-piece or penalty or a big mistake of the opponent - then the opponent had to open up to get a result and that played into their book. But the mentality to work together faded as they weren't just the young guys on one level anymore they were in 10/11/12 but some of them were stars...
Tuchel has done good work - but in difference to Klopp he is not the great mediator coach, more the tactican - and he is a "special" character. Some of the established players just weren't able to shine anymore and that made it difficult, too.
The clubs in the middle of the table always were raided by the top clubs - like the ones on the bottom got raided by the clubs in the middle. And maybe as a middle class club you cannot hold the shiny players - but you should be able to hold yourself a spine out of very good relieable players. That aren't the players that get the big money elsewhere either. And the Bundesliga and especially Dortmund seem to attract the shiny young players that are on the rise - you might not be able to hold them for years - but they bring profits.
If you look at the 6 best performing clubs in Germany behind Bayern from 12/13 to 2017 none of them was really consistent. When you take their lowest and highest point tally in this five years there is a gap of 25 points for Dortmund, 24 points for Leverkusen, 21 for Schalke, 40 for Wolfsburg, 31 for Hoffenheim. One year Champions League - the next year near the relegation ranks until somewhen in March or April. If you somewhen in the future want to rival the top clubs in Europe - not just one year as a surprise candidate when you overperform and the others have a blip - you first have to establish yourself consistently as the 2nd or 3rd team in the country because of the money the Champions League gives you. And there are more and more talents that see the Bundesliga as stepstone - recently even players from Spain or England.
In 13/14 - after Götze left - Dortmund had 5 points more in the league than they had with him.