Any argument about "quality" which only works if the project has no tests, is moot.
And two code bases sounds like a reason for more shortcuts to be taken not less to me.
And two code bases sounds like a reason for more shortcuts to be taken not less to me.
Testing means you catch regressions.
You seem to be talking about API design, which is honestly a minor concern compared to whether it works reliably.
Have you ever tried to use the Facebook API? It breaks randomly such as endpoints suddenly return 404's, and there is no followup to bug reports.
That is what I'm talking about when I say "quality".
A "perfectly designed" API that randomly breaks because someone made a change and the lack of tests means its an end-user who discovers it is worse than terrible IMO.