I think that just illustrates how risky rewrites are. Very few companies at that scale can just rewrite everything in that timeframe using that little resources. Many companies don't even have codebases that will survive a decade.
We definitely knew the risk going into it. Fortunately, it only took us 2 months to rewrite it. I think our strategy for the rewrite is directly responsible for the speed at which we rewrote it.
So you could also argue for that savings it may not be worth it. 100k is less then 1 engineer in most places (especially including TOTAL hire costs like equipment, office space, benefits, ect).
You can also argue it is much harder to hire golang engineers (or invest time/money into training engineers in go) so that that savings may not be worth it depending on the time it takes to rewrite plus the gamble of failure.
Also you need to ask could those engineers doing the rewrite have been working on up-sell features or other products that could make more money?
Would the investors (especial new growth oriented investors) care more about a increase in margins or a even larger multiple in ARR?
With node.js: 800 containers, with each container processing 250 messages per second
With golang: 340 containers, with each container processing 650 messages per second
Say each one of those containers cost $0.02/hr then that's order of $100k/year saved!