A digital signature alone cannot prevent double spending

No, going back in the list of transactions can.

Proof of work blockchain is nothing technically complicated. Nobody was doing it before because it makes no sense technically. The only reason it is used is because it adds economic value. Anyone using it for other purposes could use something else but wants VC money.

Okay then tell me how you can make a distributed ledger with global consensus without a consensus mechanism like PoW or PoS even with “no economic value”

Consensus doesn't need to involve doing hashes… there's a million distributed algorithms to do this stuff.

it's well researched https://en.wikipedia.org/wiki/Byzantine_fault

But those other solutions aren't just "using signatures".

Again, _please point out how you can have a distributed ledger with global consensus with only signatures and no other technology_