I find it hard to understand how Leaf fans can defend Dubas's contracts and contract negotiations.

Because the only contract he's put pen to paper to that he "lost" is Matthews. Nylander's contract is good, Johnsson's is good, Kapanen's is good, Tavares' is good. And Matthews is the type of player you happily overpay if it means getting pen to paper and avoiding any sort of stilted negotiation. Leafs fans defend his contracts because we pay attention to more than one signing.

showed he will cave under pressure with the Nylander deal

Time and time again this has been proven wrong but people still say it. Nylander's camp themselves told media they were the ones that made the final call. Dubas was prepared to let them sit. Dubas' biggest error with the Nylander deal was simply not getting it done earlier. The contract itself is great and there was no "caving" involved, unless you want to ignore the people who were actually involved when they say that didn't happen.

but fucking other teams with high end RFAs who will now want max dollars for short term

Dubas shouldn't give a fuck about the other teams in the league or their cap situations. And based on the fact that he's continued to be active in trades since signing the contract that supposedly fucked everyone over, the other GMs clearly aren't as torn up about it as people are pretending.