That math assumes the on-going maintenance of LedgerStore is no higher than that of using DynamoDB was.

I suspect there's now a team of technologists maintaining, securing, and operating LedgerStore.

I suspect they already had a team maintaining/debugging/operating dynamodb

Hosted services doesn't reduce maintenance costs to zero

They have teams maintaining/debugging/operating their use/consumption of dynamodb; they don't have teams maintaining/debugging/operating/securing dynamodb itself.

With LedgerDB, they now have to do both.

Optimizing to address high percentile issues (latency/error/correctness) on someone else’s code follows a bimodal pattern: it’s easy until it is impossible. Doing it on your own code is progressively more difficult and the complexity may cause you to give up, but you normally don’t hit the same wall.

Question is: Where else could they have spent those $25m? Could they have built something with more value?

Opportunity cost is a cost, too

if at uber's scale they still have enough opportunities of that scale for every one of their thousands of engineers i need to buy some of their stock

I mean, ideally you are still employing those people into the future. Plus was there other opportunities to drive value that would've been better spent?

Ideally for the people, but not a requirement. I doubt they won't be conducting more layoffs either.