No?
> Otherwise it'd been "Arguably if Go had done this years ago when go.mod did not exist, this would have had an even bigger impact" or something like this?
That’s a worst take on the same sentiment? I find your version a lot clunkier. The original sets a hypothetical stage and from that its conclusion, I think it flows better.
The stage is "years ago", and conclusion is "go.mod would not have existed and this would have an even bigger impact". My version re-arranges the sentence so that "go.mod not existing" bit is a part of the premise, not of the conclusion.
Tense agreement in English subjunctive is hard. Especially for non-natives such as me: I do parse the original statement like that and just can't bring myself to understand it otherwise.
If the comma had instead been the word "when", as suggested, this would parse the other way. It still would have been a bit awkward but would make sense.