It seems to stupid to say this because of how they've continued to recycle great talent (Gotze to Dembele to Sancho, Lewandowski to Aubamayeng to Haaland) but I doubt they can find someone as good as Sancho and Haaland. They certainly are great, but you're bound to get a dud sometime or another and when that happens your fortunes can change completely.
Reus is 31 and already you can, finally even after suffering so many injuries, see his ability starting to wain. Hummels is also going to be 32 and has already lost his speed while Witsel is also 31. These players won't give high performances for much longer so I don't think Dortmund can bank on youngsters because they won't have the older players to supplement the inconsistencies of those youngsters. What's worse is that if those youngsters don't end up being that good, then you're in for a downward spiral with not only you're experienced players gone but also no good youngster to replace them with.
Similar thing happened to Bremen in 2010, before that they were consistently in the top 4, bare one horrible season where they came mid table (but won the DFB Pokal, sort of similar to Dortmund in 14-15). They too had lost their best players but managed to replace them with great/good players (Micoud --> Diego --> Ozil, Ailton --> Klose --> Hunt). The problem though came when Ozil left and they bought Arnautovic thinking he could replace him. Suffice to say Arnautovic flopped completely and Bremen have never reached the top 6 since that season I think, that was despite still having Mertesacker, Marin and Hunt.
All it takes is one bad signing.
No offense, but that Bremen comparison and your conclusion is way off.
Bremen was always on borrowed time, because they never really translated their sporting success into (enough) financial growth, they took one risky bet after another with transfers: Almeida, Carlos Alberto and Diego were - afaik - basically flops at Porto before Bremen bought them, Özil fell out with Schalke and Boenisch couldn't break into their first team, before Werder got them, Arnautovic spent a season on Inter's bench/stands before Bremen signed him on top of that a ton of random Scandinavians. If I remember correctly their wage bill also grew to levels only sustainable with regular CL income, their transfer spending was always evenish or negative.
The comparison with Dortmund ended perhaps around 2012/13, when Dortmund proved to be more than a 1 hit wonder, translated its success into the CL, shed its debts, moved from buying underrated talents and "obscure" players to buying Reus and skyrocketed its turnover to the firm no2 in Bundesliga. Where Bremen went bargain hunting Dortmund are buying tier 1 talents. Spotting the likes of Dembele, Sancho or Haaland isn't some arcane skill or luck, even random fans know about their talent long before they move clubs. The hard part is convincing these kids that it's best for them to join YOUR club among dozens of offers, some of them from bigger clubs with better wages.
You say "all it takes is one bad signing", but you might want to look at the 16/17 window, Schürrle for €30m, Götze for €22m, for different reasons neither really had an impact. Rode for €12m was another flop, as was Emre Mor for €10m, Alexander Isak for €9m (at least if we look at what he added to the team at Dortmund - I know it looks like he'll turn out to be a fine player) and Merino for €4m (similar story to Isak really).
That's around €87m for little to no sporting impact and it didn't end the club, they actually played their best CL campaign in recent years and won the domestic cup. The club has grown enough that it can sustain a couple of bad transfers (if you look at their domestic signings there have actually been a shocking amount of them).
Reus (who is still 30 btw) has already been somewhat replaced by Brandt in terms of attacking importance, I don't know if it's age or just a bad spell, but he hasn't been
that integral to Dortmund's success this season.
Witsel has his successor lined up in Emre Can, infact tactically he probably already plays his role, as Witsel has been more attacking since they play next to each other.
Obviously Sancho won't just be replaced 1 for 1, but €150m (or whatever he will fetch) is a ton of money, enough to buy a good attacker and perhaps a great defender.
I don't know whether Dortmund will be stronger or weaker next season, there are too many transfers to be done until then, including the coaching position. Maybe they will lose Hakimi and Sancho, maybe they won't be able to compensate, but what I'm sure about is that it won't throw the club into turmoil or a downward spiral, it has become too rich and the squad too deep for that to happen and the people at the top know what to do.