These check queries are individually fast but put a tremendous load on a running app’s database. If you halve the number of queries the database has to process you’ll see more resources to process the slow queries in addition to each commit being faster.
It ultimately depends on the read/write patterns of the app, but most non-trivial rails apps do a bunch of writing AND reading.
My suggestion is less a “this will change the world forever” and more a “fairly clear win, we just have to do it”.
In a prior life I talked Sgrif into implementing it before David caused them to drop Active Record maintenance work.
I know the broad strokes but don’t have the need or mandate to implement it, plus the active record internals are pretty gnarly. Tons of modules, very little encapsulation. Very hard to know the correct place to put stuff, so stuff goes wherever. It’s gotten better from some of the work of Yehudah, Aaron, and Sage but AFAIK there’s no one on core who wants to own it right now. Eileen has done a lot of work recently, but I don’t think it’s her primary focus.
Active Record is probably the "most owned" part of Rails, that's the one with the most core members and committers with deep knowledge of it.
As for your feature request, there's https://github.com/Shopify/activerecord-rescue_from_duplicat..., and we'd like to attempt to upstream that capability in Rails next year (but as everything it may or may not happen).
If that’s been a problem for you I totally believe you, and everything you say about a fix all makes sense.